GENETICS AND BREEDING STRATEGIES:
ESSAYS FOR THE DOG BREEDER
By
Dr. Susan Thorpe-Vargas
Genetics and Breeding Strategies: Essays for the Dog Breeder
Susan Thorpe-Vargas
Portions of this book appeared as articles co-written with John C. Cargill, M.A., M.B.A.,
M.S. and D. Caroline Coile, Ph.D. Used with permission.
All rights reserved. No part of this book may be reproduced or transmitted in any form or
by any means, electronic or mechanical, including photocopying, recording or by any
information storage and retrieval system known to exist now or in the future without
permission in writing from the publisher.
Limits of Liability and Disclaimer of Warranty:
The authors and publisher shall not be liable in the event of incidental or consequential
damages in connection with, or arising out of, the furnishing, performance, or use of the
information and suggestions contained in this book.
Cataloging-in-Publication Data is available upon request from the Library of Congress
ISBN 1-929242-17-4
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Introduction
DOG BREEDING MOVES TO THE NEXT LEVEL
Why the dog opted to share his fate with man may never be known. I suspect it had
something to do with filling his stomach, but when he did so, mankind took on a moral
and ethical obligation. When individuals started to selectively breed dogs for their own
ends, their responsibility increased. How have dogs done under our stewardship? It seems
that in many cases we have “improved” Canis familaris into a genetic nightmare. Today
we have purebreds with a multiplicity of health problems that affect the quality and
longevity of their lives while simultaneously sending the cost of dog ownership skyward!
Many people believe that an excess concern for cosmetic attributes has produced
beautiful dogs that may get lost at the end of the leash. Today we have many breeds of
“designer” dogs have been created through selective breeding that cannot whelp freely or
breathe correctly. Every year billions of veterinary dollars are spent ameliorating the
effects of this tampering. Is it too late? For some breeds it may be. If they were a wild
species, certain breeds of dogs would be on the endangered list. That is why I have
written these essays. If you are a breeder, I want to make you aware of the latest
information on genetics, what you can do to avoid health problems, and screening
technology. If you are a purebred dog enthusiast, this information will help you
understand the awesome challenges breeders face.
I believe that anyone who loves his or her breed needs to know more than rudimentary
genetics. The modern dog breeder should keep abreast of the very latest information
available and take advantage of recent technological advances. What if you could screen
your dog for all sorts of genetic diseases, or double up on the probabilities of a trait’s
expression of a behavioral trait such as the herding instinct or scenting? Even though the
technology has not yet reached this stage, it is coming. With the human, canine, porcine,
mouse and other genome projects under way, the breeding game has now progressed to
the next level. Only breeders – each, acting individually – can address these problems.
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Some of this material may be heavy going, but persevere. If your eyes start to glaze over,
put the book down for a bit….and then pick it up again. The basics of genetics will be
covered as well as such diverse subjects as the origin and domestication of the dog, a
mini primer on Population Genetics, the techniques being used to discover the causes of
genetic disease at the molecular level, and the tests currently available to breeders for
genetic screening. I hope you will find this journey exciting and maybe it will encourage
you to explore further.
Some of you question the need for such a book and ask yourself why it should concern
you. Robert Jay Russell Ph.D., zoologist and breeder of Cotons de Tulear, put it
succinctly1:
“Every breeder has the ability in a free society to “determine his or her own
stopping point.” But, a single breeder’s actions may have consequences that are
far-reaching. A breed is necessarily maintained by a society of breeders. As such,
the actions of each breeder affects the actions of every breeder who dips their
brush in the gene pool and every buyer—present and future—who buys one of
these ‘works of art.’ Pragmatically (and ethically), a breeder loses his/her right to
independence and his/her ability to be independent the minute he/she puts up a
shingle that says ‘Puppies for Sale.’ ”
About myself: I thought the best way to start was to talk about my own personal
experience with breeding and my relationship with other breeders and “puppy people.”
All of us “in dogs” started somewhere, and not all of us had the good fortune to grow up
in families that were well dogged and involved in breeding, show or other dog activities.
My first Samoyed (my first ever dog) was a rescue but my family immediately fell in
love with the breed and wanted their very own puppy. We had never had a puppy before
and were not dog people. The people we got from rescue had a Christmas litter so we
bought a bitch puppy from them as a present for my son. These people were one step up
from backyard breeders as they did do some showing and they did do “rescue,” but they
had litters to make money. This Christmas puppy, call name Shisu, turned out to be my
“foundation bitch” and I was extremely lucky with my choice, only I didn’t realize it at
the time. Shisu came into heat three times between six months and a year old. My vet
told me either to breed her or fix her as this “girl wanted to be a mother.” So, I called
Shisu’s breeder who said, “I have the perfect choice for a stud, you should breed her to
her grandfather.” So we did. The litter decided to arrive on Thanksgiving Day and the
first puppy was breech. With my vet on the phone, I was walked through the process and
was able to help Shisu deliver nine puppies, one of who later died. (We think the mother
stepped on her) At six weeks, I put an ad in the paper and sold the puppies to whoever
had the money. To this day, I have no idea what happened to those puppies.
What’s wrong with this picture?

The people who sold Shisu to me should have never sold a puppy at Christmas time.
Leaving her mother and littermates was probably that puppy’s or any puppy’s most
traumatic experience of her/their life. All the turmoil and confusion associated with
the holidays is not an environment conducive to introducing a puppy to a new
household, especially a family that had never had a puppy before.
3

I had no experience with young dogs, and did not know what questions to ask.
Knew nothing about the breed-hadn’t done my “homework” and the breeder had
done no genetic testing of her dogs.

I bred a dog that was too young and had had no genetic testing done. Did not know
what were the genetic diseases common in her breed and what, if any testing was
available.

I did not carefully plan the litter, studied no pedigrees, used a sire that was to closely
related, and had not undergone any genetic clearance.

I was neither physically nor mentally prepared to help whelp the litter, nor did I have
the proper equipment, i.e., a whelping box with pig rails. I should have had an
experienced breeder with me or at the very least, assisted at a few whelpings. I put
both my mother dog and her puppies at risk because of my inexperience.
Fortunately, Shisu turned out to be a very good mother, but what if she hadn’t?

I did not have a list of qualified puppy buyers prior to the breeding of her bitch.

I placed her puppies through an advertisement in the paper. Did not require even the
most basic criteria of my puppy buyers. I did not offer any guarantees nor did I have
a puppy contract. One point in my favor: I did not sell her puppies to a pet store.

I let those puppies go out in the world with no help offered to the new owners nor
anyway to keep track of them.
I did not breed again for four years. I did a much better job the next time.
I learned that if you are a dog breeder, the purpose of having a litter is to provide yourself
with a dog that you feel will better the breed, or at least maintain a high status quo with
the best. However, every puppy you produce is not a show/performance quality dog. If
you do not recognize this, then you are seriously deluding yourself. A side effect of
producing your next show or performance dog is that you will always have pet quality
dogs to place. Your responsibility to them is just as significant as it is for the dog/s you
are keeping for yourself--maybe even more so. One should breed only dogs that have
good temperament and good health. Again I have an example of what not to do:
I bought a bitch puppy from a very well known kennel that matched the phenotype of
what I wanted to breed. This girl came from a litter of six, but only two survived.
(Warning bells should have been ringing here.) After this girl reached two years of age
and had passed her hips and eyes exams she was bred to a dog that was related to her
seven generations back. She produced eight healthy puppies, all of which survived. She,
however, developed eclampsia. Eclampsia is a life threatening condition involving an
imbalance in the blood calcium levels. She was pulled through this situation but shortly
after weaning her pups she started to get seriously dog aggressive. This behavior only
worsened when I started to show her again and she became useless on the sled team.
When I complained to the breeder, I was told to return her, and we did. Within a year of
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returning her, this dog had finished her championship and had been bred to her father.
Her breeder obviously had no moral dilemma but I believe what she did was unethical.
This girl should never have been bred again. Her life was put in jeopardy by whelping
for a second time and such close inbreeding practically guarantees an increased
probability that she would pass her poor temperament on to her offspring.
Regardless of the moral stances taken, it seems to us, there is a very real responsibility to
breed carefully to avoid creating a cadre of genetically sick dogs. Registries will be
forced by population genetics realities to modify their views of what constitutes purebred
dogs. Breeders need to rethink their understanding of the benefits of line breeding and
other such tight inbreeding schemes in favor of assortive matings to preserve genetic
diversity. Those involved in breeds with few founders will run up against genetic reality
sooner than some others. There is near certainty that there will be a day of reckoning
where the genetic choices made in the past will determine the dogs of the future.
NOTE: Words that are in bold type when they first appear are in the Glossary.
Chapter 1
EXTRAORDINARY DIVERSITY
Have you ever wondered at the extraordinary diversity in the appearance of various dog
breeds? How is it that a Yorkshire Terrier can be the same species as a Bullmastiff, or a
Pug be related to a Saluki? What are the factors that have led to this incredible range and
variety in appearance, not to mention behavior and temperament? It is not simply a
question of phenotype vs. genotype, or dominant vs. recessive genes. Let’s begin the
journey by looking at how dogs evolved into the companion we know and love today.
WHY SO MANY DOG BREEDS?
About 60 million years ago a small weasel-like animal lived in many parts of Asia. This
ancestor of all modern day canids (dogs, jackals, wolves and foxes) was called Miacis.
Cynodictis, the first true dog-like canids are thought to have descended from Miacis
about 30 million years ago. This line eventually split into two branches, one in Africa and
the other in Eurasia. The Eurasian branch was called Tomarctus and was, until recently,
thought to be the progenitor of wolves, dogs, and foxes. However, new research has
called this theory into question with a recent paper indicating now that the wolf is the
domestic dog’s only direct ancestor and that a recently shared ancestry with the fox and
jackal is unlikely. 2 This somewhat controversial paper also asserts that the first
domestication of wolves may have taken place as long as 100,000 years ago. The actual
time that such domestication occurred, of course, cannot be settled based solely on
mtDNA analysis. Indeed, the new data does not clearly support that the dog is descended
5
from the wolf. Neither do the fossil remains. A case can still be made that they coevolved
from a common ancestor.
Research now suggests that the domestic dog line began to diverge from the wolf after
the first wolf became domesticated. Over time, groups of wolves became adapted to a
niche that made them ultimately better suited to domestication at some point as early as
100,000 years ago to as late as 14,000 years ago. The actual timing remains in dispute
since the fossil records are not consistent enough to pinpoint an exact period of time.
However it has been well established now that different domestication events did occur
from multiple populations by researchers such as Robert Wayne3. This makes sense as
both wolves and humans coexisted over a wide geographical area and it is likely that
multiple domestication opportunities would have arisen. These multiple events in various
parts of the world accentuated the diversity we see in dogs today.
As hunter/gatherers, humans would have found dogs very useful. Then, about 8,000 years
ago, humans turned to a more settled way of life. This is when severe selection for
specific behaviors and traits became important and ‘modern’ breeding practices started.
And so it begins – dogs bred for many reasons, from companionship to guarding abilities
and so on.
The concept of a “pure” breed is a relatively recent one; further back, local dog
populations consisted of similar looking dogs bred for a specific purpose. Although there
were some exceptions, the dog breeders of that time did not hesitate to breed a dog of one
type to a newly arrived dog from another area. Thus, up until the 19th century the various
dog breeds were more often than not strains of closely related and similar looking dogs
that as a population had a great deal of genetic diversity. Only dog populations that lived
in geographic isolation approach today’s purebreds in terms of a restricted gene pool.
In searching for cultures without dogs since pre-historic times we come up empty handed.
Thus we find early recognizable breeds coming from the Middle East, Africa and Asia.
The Middle Eastern coursing hounds had become well established no later than 2,000
BC. The Basenji, a hunting breed of the African savannah, may predate the dogs of the
Pharaohs. In the Far East, isolated areas such as Tibet and Mongolia produced a number
of still extant breeds of ancient origin. Malta was occupied as early as 3,500 B.C and the
dog brought to Malta may have had earlier Egyptian origins. The point here is that since
relatively early times in recorded history, there has been a tremendous diversity in dogs.
Contrast the Roman Mollosus – a mastiff-like creature (or what we think it looked like)-with the Maltese or the Tibetan terrier or the Lhaso Apso, and it immediately becomes
obvious that there may be no “standard” dog. The tiny kingdom of Tibet, produced many
different breeds, some now probably extinct, but which include the following breeds and
their ancestors: Kuvasz (before Hungary); Lhaso Apso, Tibetan Terrier, Tibetan Spaniel,
Tibetan Mastiff, just to mention a few familiar to Western dog fanciers. Even dogs we
think of as “English” such as Mastiffs have had ancient origins. Recognizably mastifflike dogs can be seen on Egyptian monuments circa 3,000 BC. They were in China circa
1100 BC and eventually went to England with invading Roman forces in the first century
AD. We can safely say that certain dog strains have been breeding true for a very long
time.
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NEOTENY – MORE FUEL FOR DIVERSITY
There is a saying among dog breeders that “All puppies look alike.” Newborn puppies of
different breeds, except for size and of course color, look remarkably alike. How is it that
they grow up to look so different from one another? The vast array of physical and
behavioral differences in dogs is probably not due to selection for each individual trait,
but more likely to selection for groups of traits that are all similarly affected by the same
hereditary mechanisms. One such mechanism is the regulation and timing of
developmental processes. Selection for one trait affected by developmental timing could
inadvertently select for other traits also thus affected.
It is very likely that this process has played a vital role in the initial domestication and
later diversification of dogs. As animals mature, they pass through different stages, each
uniquely adapted to its particular circumstance. In wolves, neonates and juveniles are
dependent upon parents to care for them, and they are extremely successful at eliciting
that care. In comparison to adults, they are relatively tame and subservient. Wolves (or
the wild ancestors of wolves and dogs) that tended to retain these immature qualities of
tameness and subservience into adulthood would have been favored by early humans and
would have formed the core of primitive domesticated dogs. This retention of immature
characteristics in adults is known as neoteny. By choosing the individuals to reproduce
that showed the favored immature behavioral qualities, concurrent selection for other
juvenile traits -both wanted and unwanted--may very likely have occurred, laying the
basis for the diversity seen in dogs today.
The rounded head and shortened muzzle of some breeds is reminiscent of the neonatal
wolf. Floppy ears, too, are a neonatal wolf trait. The dog’s smaller head, brain, and teeth,
in comparison to wolves, are comparable to those of the immature wolf. Many of the
herding, hunting, and playing behaviors humans have found so useful and entertaining
can be found in immature forms of hunting behavior in wolves. Barking is rarely seen in
adult wolves, but is a trait of juveniles---as well as adult dogs. Further crossing of dogs
showing differing degrees and influences of neoteny could produce novel combinations
of adult and immature characteristics, so that domestic dogs may be regarded as a blend
of immature and adult characteristics. Sometimes this creates problems for the dog
breeder as in the case of toy breeds with disproportionately large eyes. The eye seems to
be relatively immune to neoteny and is therefore difficult to reduce in size through
selection in contrast to body and skull size, both of which have been induced to retain
immature dimensions.
Now, let’s take a look at some of the basic concepts of Genetics that every breeder needs
to know. Then we'll discuss the mechanics of inheritance in Chapters 2 and 3.
Chapter 2
A GENETICS PRIMER
Are you mystified by the genetic code? Do you blanch at words such as allele,
dominant, and codon? Do you think of microsatellites as small orbs circling the Earth?
The time is coming when such words will be part of the everyday vernacular of dog
breeders. The study of genetics has previously been the domain of specialists, but it is
rapidly becoming part of the responsible breeder’s repertoire. The study of genetics is
much like learning a foreign language. It really isn’t all that difficult to become
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conversational once you master the rules and become comfortable with a new
vocabulary.
GENETICS 101 – BASIC CONCEPTS
Each cell within the body is composed of cytoplasm, a jelly-like layer of material that
surrounds a nucleus. Within the nucleus are a number of threadlike chromosomes that are
almost entirely made up of two kinds of chemical substances: nucleic acids and proteins.
Nucleic acids have at least two functions: to pass on hereditary characteristics and to
trigger the manufacturing of specific proteins. The two classes of nucleic acids are the
deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA). DNA, the genetic
building block, is made up of substances called nucleotides, each of which consists of a
phosphate, a sugar known as deoxyribose and any one of four nitrogen-containing bases.
These four nitrogenous bases are adenine (A), thymine (T), cytosine (C) and guanine (G).
Canine DNA is about 6 billion nucleotide pairs long. Each base is attached to a sugar
molecule that is linked by a hydrogen bond to a complementary base on the opposite
strand. These bases are complementary because only adenine pairs up with thymine, and
only cytosine pairs up with guanine; thus the pairs are AT and CG.
In all mammals, the DNA molecule appears as two complementary strands that are
wrapped around each other like the railings of a spiral ladder, known more formally as
the double helix of Crick and Watson. 4 The strands (sides of the ladder) are composed
of alternating phosphate and sugar molecules. The nitrogen bases, joining in pairs, serve
as the rungs. The two strands are held together by weak electrical bonds between the
bases on each strand/rung, thus forming base-pairs. Each strand has its own polarity
opposite of the other. Thus if you turned the strands upside down, the picture would not
change. An easy way to visualize the opposite polarity aspect of the two chains is to think
of two identical snakes intertwined around each other but facing opposite directions (head
to tail and tail to head). Thus, each half of the double helix serves as a genetic template of
its complementary half.
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2-1
Before a cell can express a particular gene, it must first transcribe that specific part of the
DNA into messenger ribonucleic acid (mRNA). This is similar to the formation of a
complementary strand of DNA during cellular division, except that RNA contains uracil
(U) instead of thymine as one of its four nucleotide bases. In the process of transcribing
DNA into mRNA, all the T bases are converted to U bases. These bases C, G, A, and U
are the alphabet of the genetic code. A sequence of AGT in the coding strand of the DNA
thus produces a sequence UCA in the mRNA.
Think of codons as three-letter “words” identifying the bases DNA uses to specify
particular amino acids as building blocks of proteins. Normally, codons signal the
initiation of a protein chain, its end or a particular amino acid. For example, CUU stands
for the amino acid leucine. CUA, CUG, and CUC also “code” for leucine, so there is
some redundancy in the system. Notice in this example that it is only the last base that is
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different (U vs. A vs. G vs. C). The term degeneracy is used when a change in a base
does not affect the amino acid being added to the polypeptide.
BREAKING DOWN THE GENE
What is a gene? A gene is the basic unit of inheritance. Genes contain a set of directions
for producing a bit of RNA, a protein or a polypeptide. If all goes well, a complete set of
genes - one half from each parent - is inherited. If the two copies of each gene are exactly
alike, the progeny are homozygous at that locus. When homozygous, only one form of
the gene will be passed on. If the gene inherited from one parent is different from the
gene inherited from the other, the progeny are heterozygous. In this case, there is an
equal chance that one or the other form of the gene will be passed on. Different forms of
the same gene are called alleles.
In the dog, the various genes are located among 78 different chromosomes. What is not
known is how many genes exist, although a rough estimate has been made that there may
be as many as 100,000 or as few as 30,000. It is also not known where on the various
chromosomes specific genes are located. In fact, scientists have just recently karyotyped
the canine. This provides the ability to differentiate between specific chromosomes. This
will be valuable information when scientists are finally able to map the canine
chromosome. Such a genetic map will not only allow the determination of the position of
genes relative to each other, but also will reveal their approximate distance from each
other.
THE CELL CYCLE
Mitosis is the process of one cell splitting and becoming two cells. This act of division is
the result of a series of events known as the “cell cycle,” which consists of several
distinct phases or stages. The resting or quiescent state between cellular divisions is
called the “G0” stage. “G1” is the phase in which all the cellular proteins needed for
mitosis are made and is the first control point where the cell must “decide” to move on to
the next stage--the “S” phase. During the “S” phase, the cell’s genetic material is
duplicated so each of the daughter cells is genetically identical to the parent cells--unless
something goes wrong. The S phase is so named because this is the point during which
new DNA is synthesized. The period from the end of the S phase until the actual division
of the cell is known as the “G2” stage and is the second control point during which a
decision is made whether or not to actually undergo division. Mitosis is further divided
into prophase, metaphase, anaphase, and telophase. The stages G1, S and G2 constitute
the interphase.
In a process called meiosis, germline cells--sperm and eggs- go through one more
division in which their DNA is not duplicated. This leaves them with only half the normal
number of chromosomes that somatic cells have. Somatic cells are all the cells of the
body except the germline cells. Once these two cells combine to form a fertilized egg or
zygote, they then have the proper amount of DNA--half from their mother and half from
their father. This is important because mutations in the germline cells are passed on to the
offspring, whereas those that occur in somatic cells only affect the individual.
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2-2 Stages of the cell cycle
GENE MUTATIONS
Mutations (changes in the genes) are caused by a variety of mechanisms. Some of the
most common are mistakes made when the organism’s DNA is replicated prior to a cell
dividing. Although there are body system safeguards in place to prevent this from
happening, nothing is foolproof, and eventually over time, failure to replicate DNA
accurately will occur.
Likewise, errors can occur all along the pathway that leads to the translation of
messenger RNA into a specific protein. These errors can occur spontaneously or be the
result of exposure to natural and/or man-made mutagens. Certain chemicals or exposure
to certain types of radiation can cause genetic changes. What is important to remember is
that these mutations are random events with respect to their adaptive potential. In other
words, they will happen independently of whether they have beneficial or harmful
consequences. More often than not these mutations are harmful, as they are changes to
the make up of a living organism. Just how harmful depends upon the type of mutation
that occurs and the environment in which they occur. Most mutations fail to thrive,
reproduce or survive and thus are not passed on to successive generations. There are
several kinds of gene mutations, each having a unique range of potential effects. This is
important to recognize because many genetically transmitted diseases result from a
specific kind of mutation. Each of these forms of mutation is the result of the organism
failing to reproduce its DNA accurately all of the time and subsequently passing these
genetic changes to successive generations.
Base-Pair Substitution Mutations
The result of this type of mutation can range from no effect at all to one that has severe
consequences to the affected organism. Remember DNA is made up of four different
nucleic acids: thymine (T), adenine (A), guanine (G) and cytosine (C), and remember that
thymine always pairs up with adenine and guanine always pairs up with cytosine. Hence
the name base-pair mutation. Sometimes when the DNA strand is being replicated the
wrong base is inserted. This can result in a different amino acid being added to the
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protein being made. If the essential biological function of that protein is not changed then
there is no detectable effect. However, if the substitution affects the active site of an
important enzyme or changes it’s three-dimensional shape, then it modifies the
fundamental nature of the protein. If this occurs along an essential metabolic pathway,
the results can be disastrous.
The most unfortunate result of a base-pair substitution is when this mutation codes for a
stop codon. Keep in mind that a codon is that portion of the messenger RNA that codes
for a specific amino acid. A start codon serves rather like a capital letter indicating the
start of a sentence. A stop codon is one that does not specify an amino acid, and serves
much as a comma or a period punctuating the genetic message. If, by chance, a mutation
produces one of the stop codons, than the process of making the protein is terminated.
An example of this type of mutation is the one that leads to a form of progressive retinal
atrophy (PRA) in the Irish Setter. A substitution of an A for a G produces the stop codon
(TAG) that replaces the normal codon for the amino acid tryptophan (TGG). This
prevents a protein called PDEB (phosphodiesterase beta) from being produced in its fulllength form. The shortened protein is unstable and is degraded within the retinal cells in
which it is needed. The lack of this protein causes the retina to degenerate, resulting in
blindness in those Irish Setters that have two copies of the mutant gene, and no normal
copy.
Frameshift Mutations
In the normal cell replication process, DNA is transcribed into messenger RNA, which in
turn is translated into a series of amino acids. This always occurs in a specific manner,
i.e., it always begins at a definite spot and it is ‘read’ in multiples of three and in a
particular orientation along the length of the strand of DNA. This is called a reading
frame. If there is an addition or deletion of one or two base pairs, then the result is often
a very altered sequence of amino acids in the final protein product resulting in what is
termed a frameshift mutation.
An example of this is the mutation that leads to an inherited form of anemia in Basenjis.
A deletion of a single nucleotide in the 433rd codon of the gene encoding a protein called
PK (pyruvate kinase) causes a shift in the reading frame. The misformed and shortened
protein (a new stop codon is ultimately encountered) is unstable in the red blood cells that
carry oxygen throughout the body. The lack of the PK causes the red blood cells to
slowly be destroyed and results in the anemia.
Splice-Site Mutations
Molecular geneticists used to think that all of the DNA coding for a particular protein
was continuous until they started to look at more complex organisms. What they found,
in these types of cells, is that the DNA that makes up a gene is often distributed in
discontinuous sections called exons, interspersed with long segments of non-coding DNA
known as introns. These sections are transcribed into messenger RNA along with the
exons, but before the RNA is translated into a protein they are ‘edited’ or ‘spliced’ out. A
change of even a single nucleotide in one of the exons of the gene can cause a shift or
alteration of the splice-site.
A genetic disease that affects Dobermans is a perfect illustration of this type of mutation.
von Willebrand disease is a bleeding disorder that effects the animal’s ability to form
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blood clots. Other breeds also have this disease, but what had perplexed those doing von
Willebrand research was that Dobermans appeared to have a milder form of the disease.
The discovery of a splice-site mutation that codes for von Willebrand factor has cleared
up their mystery. George Brewer MD of the University of Michigan suggests that one use
the following analogy in order to explain how the mutation functions5. Imagine that a
freight train is supposed to go from point A to point B along a railroad track. Somewhere
between A and B is a spot where a sidetrack goes to point C. Normally, the train never
goes to point C because the switch, that connects the two tracks, is never thrown. If the
switch is broken (the mutation) then the lock that prevents the track from connecting to
point C is no longer effective. The switch can now toggle back and forth, sending some
trains to point B and some trains to point C. In affected Dobermans, the defective switch
sends the train to the wrong destination about 90-95% of the time, the train rumbles over
the cliff and is never heard from again (i.e., the proper protein is never made). However,
sometimes the switch jiggles the right way and the train ends up at the normal destination
and the proper protein is made. If both copies of the gene are mutated, then each gene can
make the right protein about 5 to 10% of the time. Affected Dobermans are thus
producing von Willebrand factor at least some of the time and so their symptoms are not
as severe. A mystery explained.
CHROMOSOMAL ABNORMALITIES
Other types of mutation occur during cellular division because of chromosomal
abnormalities. Let’s review how chromosomes are normally duplicated during cellular
division. Keep in mind that the prophase is the first stage of cell division. The nucleus
swells and the chromosome becomes visible. During interphase the DNA has been
duplicated and consists of two linked (sister) chromatids held together at a centromere.
A structure called a centriole appears and moves towards the opposite poles of the
nucleus. The next stage is called the metaphase. During this period the spindle fibers are
formed and are attached at the centromeres and the chromosomes line up along the
equator. This is the very best time to examine the complete set of chromosomes within a
cell. If a cell is “fixed” at this point in the cycle and stained with special dyes, a
cytogeneticist can determine if there is the correct amount of DNA, any deletions or
other abnormalities and the sex of the individual. This “picture in time” is called a
karyotype
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2-2 Canine Karyotype
Following the metaphase is the anaphase. During this stage, the sister chromatids separate
and migrate to opposite ends of the cell, the nuclear membrane has disappeared and the
cell elongates as the diameter of the cell decreases at the equator. At telophase a new
membrane is formed about the two new cells, the chromatids (now called chromosomes)
uncoil and the nucleus is reformed. In sum, chromatids are just compacted chromosomes.
Before looking at the different types of chromosomal abnormalities let us first discuss
some of the terms used to describe them. Chromosomes are either sex chromosomes, in
mammals these are the well-known X or Y chromosome, or autosomes. Autosomes are
any chromosome that is not a sex chromosome. Chromosomes can be divided further into
metacentric, submetacentric and acrocentric. These terms describe the position of the
centromere within the chromosome. Metacentric chromosomes have their centromere
near the center of the chromosome. Those chromosomes whose centromeres are slightly
off-center are referred to as submetacentric, while acrocentric chromosomes have their
centromere located close to one end. Dog chromosomes are mostly acrocentric. The small
“arm” of the chromosome is referred to as p for “petit” and the longer arm is called q –
the next letter in the alphabet. Another naming protocol is to designate regions and bands
from the centromere outward. Depending on which staining techniques are used, banding
patterns are seen that are characteristic for each chromosome pair. Thus, the designation
7q31.2 refers to the long arm of chromosome 7, region 3, band 1, sub-band 2.
Numerical chromosomal abnormalities
Normally dogs have 39 chromosomes in their germline cells, i.e., sperm and eggs.
Germline chromosomes are haploid, i.e. they contain one copy of each chromosome.
Somatic cells are diploid – they contain two homologous copies of each chromosome.
A failure of the chromosome to separate properly during cell division (nondisjunction)
can lead to a decrease or increase in the number of normal chromosomes. For example:
Triploidy is the presence of three haploid sets of chromosomes, instead of two.
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Monosomy is the situation where a diploid cell, in which normally one or more of the
homologous chromosome pairs is represented, only has one chromosome of the pair.
Trisomy is the term used which indicates the presence of an extra whole chromosome.
Each canine somatic cell usually has 78 chromosomes (2x39), but in trisomy, this is
increased to 79. Down’s syndrome in humans is caused by this anomaly as there is an
extra chromosome number 21.
Structural anomalies
A chromosomal deletion occurs when part of a chromosome is missing. The damage that
can occur depends upon how big of a piece is missing and where the deletion occurs. A
chromosomal duplication happens when a section of the chromosome is reproduced
twice. Depending on what section is duplicated there can be extra sets of genes present
that can cause birth defects or developmental problems. A chromosomal ring occurs
when the q and the p ends stick together. This can cause loss of information and/or cause
problems when the cell divides. A chromosomal inversion is caused when there are two
breaks in one chromosome and the area between the breaks are turned around and
reattached. If the break includes the centromere, it is called a pericentric inversion. If not,
it is a paracentric inversion.
Chromosomal translocations are rearrangement of a chromosome in which a segment is
moved from one location to another, either within the same chromosome or to another
chromosome can be balanced or unbalanced. A balanced translocation occurs when the
same piece of chromosome, say the q arms of two different chromosomes, are broken off
and attached to the other chromosome. No genetic material is lost, it is just on a different
chromosome. This should not cause problems with the individual but could with that
individual’s progeny. An unbalanced translocation, occurring in a germline cell, results in
3 copies of a section of chromosome in one cell and only one copy in the other. Both
trisomy and translocation are implicated in canine cancers. The other chromosomal
anomalies are well characterized in human disease but not in the canine.
Chapter 3
WHAT YOU GET IS NOT NECESSARILY WHAT
YOU SEE!
The age-old problem for dog breeders of course is that the characteristics they are trying
to breed for do not always materialize in a litter of puppies. Or some unwanted
characteristic keeps appearing that even “careful” breeding cannot eliminate. Why does
this happen and what can be done to eliminate at least some of the uncertainty a breeder
faces?
GENE EXPRESSION
Keep in mind that the number of genes in the entire dog genome has been estimated to
range from 30,000 to 100,000. Thus the breeder is dealing with a very large number of
variables. Remember that every gene is the blueprint for either regulatory RNA, a protein
or a snippet of amino acid called a polypeptide, but these gene products are not being
made all the time nor at the same time. Regulation of expression involves turning genes
on and off at various intervals and in a particular temporal sequence.
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The first step in gene expression begins with transcription. This is the process of
copying a DNA sequence called the template, into a single strand of RNA known as the
primary transcript. This operation is initiated by an enzyme called RNA polymerase.
Genes come in two types, structural and regulatory. RNA polymerase is a protein that
is coded for by a regulatory gene. Transcription starts when this enzyme binds to a
special region at the start of the gene called the promoter and continues until it reaches a
terminator sequence. The first point of control in this process is therefore the binding of
the enzyme to this specific site.
While all genes are present within a cell, only certain genes within any cell express
themselves through a process called cell differentiation. Have you ever asked yourself
why are the cells in your fingernails only producing fingernail proteins and not, lets say,
eye proteins? The simple answer is that all the other genes in the cell, except those coding
for fingernail proteins are somehow turned off.
In the process of maturation, a cell progressively and irreversibly becomes more
committed to a certain line of development. One of the ways scientists think a cell can
‘remember’ what it has decided to be seems to depend on the chromosomes. Control of
gene expression is the result of regulating transcription initiation. Chromosomes play
a unique role in this process. It is possible to see cellular DNA only during certain phases.
Most of the time it exists in a relatively uncondensed form and it is only during this
dispersed phase that transcription can occur. However, even during this stage, some parts
of the chromosomes stay tightly wound up and condensed. The part that is unwound is
called euchromatin and it is transcriptionally active. The part that stays condensed
cannot be transcribed because the transcriptional factors are physically unable to get to
the DNA.
There are two types of inactive chromosomes. One is called constitutive
heterochromatin and it is always transcriptionally inert. The other is referred to as
facultive heterochromatin and it varies in a tissue-specific manner. So, depending on
which cell type it is, large blocks of chromosomes are physically prevented from being
transcribed. This constitutes regulation at rather a gross level, a finer aspect of control
exists in the specific sequence of the DNA itself.
Another form of gene regulation can result in an entirely new protein being made or, in
some cases, no gene product at all. This can happen through the selection of alternative
transcription initiation sites or optional splice sites. An additional control mechanism has
been suggested by the processing of messenger RNA. It is mRNA that is actually
translated into the final gene product. Whether or not messenger RNA makes it out of
the nucleus so that it can be made into a protein, or how long it lasts before it is degraded,
would definitely affect the final gene product. However, research has barely begun on
these topics, so we will leave it for now to discuss another pathway to phenotypic
differences.
ALL ALLELES ARE NOT CREATED EQUAL – DOMINANT AND
RECESSIVE GENES
Control of gene expression also depends on how genes interact and their alternative
alleles. Because chromosomes are present in pairs, it stands to reason that the genes on
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them are also present in pairs. Genes in corresponding locations on homologous
chromosomes are called homologous genes, and when these homologous genes can code
for different proteins, they are called alleles. Sometimes we are aware of only two
possible alleles for a particular gene, but often several possible alleles exist. Only two at a
time can be present in one individual, however.
The possibility of having either identical or nonidentical allele members of a pair creates
an array of different ways these alleles can interact. When one allele can completely
mask the presence of the other it is termed complete or simple dominance. If both
alleles can be expressed equally then codominance occurs. Sometimes the end result
may be intermediate between the products of the two alleles generating incomplete
dominance.
The gene can be considered a small business with two partners. Sometimes both partners
share the same desires, just as both alleles may code for the same products. This is the
situation with homozygous alleles. Sometimes partners, and alleles, don’t agree, such as
with heterozygous alleles. Heterozygosity can have several outcomes. As in any
“partnership”, decision making can take several forms. In some cases one partner (the
dominant allele) calls all the shots, regardless of the wishes of the other (recessive allele).
In genetics this is known as simple dominance. In other partnerships, compromise is the
order of the day, and when the two partners are not in agreement, they settle on an
intermediate solution (incomplete dominance). In yet other partnerships, both members
go ahead and do what they want to do regardless of what the other does. In genetics such
a solution is termed co-dominance.
Simple dominance
Dog breeders sometimes fall into the trap of assuming a trait is due to a dominant allele
because “even after being hidden for generations it just popped back out…I can’t seem to
get rid of it”. In fact, they have put their finger on the signature of the recessive allele.
Consider the case of black versus liver hair color. A single dominant allele (B) codes for
black pigmentation. Dogs that are either BB or Bb will be black and indistinguishable
from one another. The “science speak” way of describing this is to say that the genotype
is different but the phenotype is the same. In the case of two recessive alleles, bb, liver
color result. If two liver (bb) dogs were bred together, they could only produce liver
offspring. If two black dogs were bred, the possibility exists that both of those dogs could
be heterozygous (Bb) and produce a bb offspring that would be liver--not because the
liver was dominant, but because it was recessive and thus hidden in the parents. A trait
caused by a dominant allele can be traced directly from one ancestor to the next through a
pedigree, although, as we will see later, other genes can also act on the dog’s color to
possibly modify or obscure it. Not all traits are inherited in this manner, however. In fact,
most traits do not show simple dominance.
Incomplete dominance
In contrast to simple dominance, in which two alleles produce three possible genotypes
but only two possible phenotypes, incompletely dominant allele pairs produce three
possible genotypes and phenotypes. The merle coat color pattern (found in breeds such as
the Australian Shepherd, Dachshund, and Collie) is an example of an intermediate
phenotype created by two non-identical (M and m) alleles. Dogs that are mm have
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“normal” non-merle coat colors determined by genes at other locations. Dogs that are
Mm display the classic merle color, in which areas of the coat have loss of normal
pigmentation, resulting in the appearance of flecks or patches of normally colored hair
interspersed among lighter hair. Dogs that are MM have greater pigment loss, may be
nearly white, and very often have visual and auditory problems that are pigment related.
Breeders thus usually discourage merle to merle breeding, since ¼ of the progeny of a
Mm x Mm breeding would be MM—and therefore likely to have vision and hearing
problems. Instead, taking advantage of incomplete dominance, merles (Mm) are best
obtained by breeding non-merles (mm) to merles (Mm), resulting in litters consisting on
average of 50% Mm merles and 50% mm non-merles. Two simple tests can determine if
a trait is incompletely dominant. For one, crosses between two different parental types
should always result in the intermediate type. For another, crosses between two
intermediate types should result in both intermediate as well as parental types.
Co-dominance
In yet another example, other alleles code for products that can both be distinguished in
the individual. The most common examples of this codominance are usually found in
certain blood proteins expressed in both people and dogs. Perhaps the simplest and most
familiar are human blood groups. In humans, three possible alleles exist: A, B and O. A
and B are dominant over O, but are codominant with each other, thus resulting in the AB
blood type.
Penetrance and Expressivity
Just when early researchers thought they had dominant and recessive inheritance clearly
defined, they kept coming across cases where an allele that should have been expressed
wasn’t. The most obvious were in identical twins that weren’t quite identical. One would
exhibit a trait known to run in that family while the other would not, yet they were
identical in all other respects. This is known as variable penetrance. Related to this is the
concept of variable expressivity, where both twins would share the same trait, but one
would have a more pronounced version of it than the other. Two dogs that both carry the
same alleles for spotting may have very different spotting patterns. For some reason some
alleles will not always be expressed, or will be expressed to varying extents, in an
individual that should normally express them. For the breeder, these two phenomena can
make tracking the hereditary pattern of a trait more complicated.
Pleiotropis
Some genes affect widely disparate traits. Chinese Cresteds come in a hairless and
powderpuff varieties, with the hairless caused by a single allele H. In fact, this is a
homozygous lethal allele, because dogs with HH die before birth so hairless dogs are all
Hh. The H allele not only results in hairlessness, but also in tooth abnormalities, which is
why allowances are made for hairless Cresteds with missing teeth. Because these two
traits are pleiotropic i.e., the effects of one allele, they cannot be separated and one must
always go with the other.
In addition to the interactions that occur between alleles at the same locus, interactions
can also occur between alleles at different loci. Examples of traits involving different loci
include the concepts of phenocopies, linkage, epistasis, and perhaps most important,
polygenic effects.
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Phenocopies
Sometimes two dogs will seem to share the same trait but in fact the trait is the result of
totally different genes. White dogs can result from the alleles for extreme white spotting
(basically a spotted dog without any spots showing) or from a dog with several alleles at
different loci for factors that make the coat pale (basically a cream dog that is so pale it
appears white). The eye disease, Progressive Retinal Atrophy (PRA) exists in many
breeds, sometimes appearing clinically identical, even though the genetic cause is
different. This means that even though PRA is recessively inherited in affected dogs,
crossbreeding different breeds may yield normal offspring because unique genes in the
two breeds cause the disease. (If an affected dog of breed A is pp RR, and an affected dog
of breed B is PP rr, then their offspring would all be Pp Rr, and appear normal). For
example, Irish Setters and Collies have genetically distinct forms of PRA.
Epistasis
Not only can alleles interact with other alleles at the same locus, but in some cases, with
alleles at other loci. While dominance can be considered an intralocus interaction,
epistasis can be considered an interlocus interaction. The simplest case of epistasis
occurs when the presence of one trait effectively masks the presence of another trait.
Such an example occurs with Labrador Retriever coat colors. At the B locus, the
dominant B allele codes for black fur (BB or Bb) and the recessive b allele for chocolate
fur (bb). However, at a totally different locus, E, the presence of the dominant E allows
either black or brown fur (according to what is determined at the B locus), but ee restricts
the formation of any dark pigment, thus resulting in a yellow dog no matter what is coded
for at the B locus.
Another form of inheritance (above the genes) includes a phenomenon called
epigenetics. Imprinting is an example of this. Normal development requires genes to be
inherited from both parents. Genetic imprinting is the situation where the expression of
the gene is determined by which parent you inherited the gene from. Some disease genes
are expressed as an entirely different disease, depending upon whether you inherited the
gene from your mother or father. Imprinted genes occur in those regions of specific
chromosomes with allelic differences in transcription and methylation. A mutation
within an imprinting region can cause genetic abnormalities. In humans, Prader-Willi
syndrome (PWS) and Angelman Syndrome (AS) are examples of two different disorders,
with entirely different phenotypes, that are caused by either a maternal or paternal
deletion on chromosome 15 or when the inheritance of both chromosomes is from just
one parent (uniparental disomy). Only paternal deletions are seen in PWS and only
maternal deletions are seen in AS. When both copies of chromosome 15 are inherited
solely from the mother the disease presents as PWS and just the opposite when AS is
seen.
Polygenes
The problem dog breeders have with using ideas of dominant and recessive genes in
breeding dogs is that most traits don’t appear in discreet intervals, but instead are
continuously distributed over a range of values. This is also called a quantitative trait. For
instance, dogs don’t come in just short, medium, and tall, they come in all sizes. Even
within a breed, height is normally distributed in a bell curve. This is because many
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important traits are the result of many pairs of genes acting together. In these cases, the
extent of a trait is determined by gene dosage, which is the number of particular alleles
present in a genotype.
Imagine that height is controlled by incompletely dominant alleles at three different loci,
A, B, and C, with A+, B+, and C+ all coding for an additional half inch of height. A dog
with the genotype A+A+, B+B+, C+C+ would be three inches taller than one with the
genotype AA, BB, CC. In fact, 27 different genotypic combinations are possible in this
example, resulting in seven different heights. The more loci involved, the greater the
number of possible genotypes and phenotypes, until the phenotypes become so numerous
that they appear to be continuously distributed. This blending is further influenced by
environmental factors. In fact, all quantitative traits have an environmental component. A
good example of this in the canine would be hip dysplasia. Hip dysplasia is a disease that
is known to be influenced by the amount and quality of food that the puppy eats.6
Heritability
The fact that gene expression can be influenced by the environment should not be
surprising. The value of how much the expression of a trait is genetically determined is
called heritability. This value ranges from 0 to 1 and the higher the value the more likely
that selection will play a role in the inheritance of that trait. What should also be stressed
is that the heritability of a trait in one group cannot be compared to that of another group.
Heritability is strictly a function of a particular population, at a specific time and place.
One should also be aware that no matter how high the genetic heritability of a trait there
will always be some environmental component in the expression of that trait.
Linkage and Linkage Disequalibrium
In a highly inbred population, genetic defects can become fixed rather rapidly if they
happen to be on the same chromosome as a gene that codes for a desirable trait. The
closer they are physically on the chromosome the tighter they are ‘linked’. These genes
and their respective alleles will be inherited together unless they become ‘unlinked’ in a
procedure called crossing over or recombination. This is a process that occurs during
the formation of gametes. At that time, homologous chromosome pairs exchange
segments of their DNA structure. Such closely linked genes are said to be in a state of
linkage disequilibrium. When a breeder selects for or against a specific gene trait, he or
she may also inadvertently be choosing other traits which are located on the same
chromosome. One should remember this when making a breeding decision. Severe
selection pressure against an unwanted trait could result in throwing the baby out with the
bathwater and the permanent loss of a necessary or desirable attribute.
Sex Linkage
A special case of linkage exists when genes are located on the sex chromosomes. Unlike
the other 38 pairs of chromosomes, the sex chromosomes are not always paired in a
homologous fashion. This is simply because sex is determined by whether an individual
has two X-chromosomes (XX=female) or an X and a Y chromosome (XY=male). The Y
chromosome is a very small chromosome and until recently there were doubts that any
significant information was contained on it. Those genes found on the Y chromosome
that have been identified, code specifically for male traits. The X chromosome is larger
and is known to carry on it genes that code for several traits important for males as well
20
as females. Genes on the X chromosome are not matched by genes on the Y
chromosome, negating the possibility of allelic pairs.
In the male, whatever alleles are on his single X-chromosome will be expressed (a
condition known as hemizygous). In the female, the situation is different than what is
seen in the autosomal (nonsex) chromosomes. For many years it was assumed that Xlinked alleles acted just the same as autosomal alleles. They don’t. Instead of acting in a
standard dominant/recessive way, these alleles act more like codominants. In placental
mammals one of the two X-chromosomes is randomly inactivated in each cell of the
body. The remnants of these inactivated chromosomes can be seen as dark spots, called
Barr bodies, in almost every cell of a normal (XX) female, but not in normal (XY) males
because they must have a functioning X chromosome. However, researchers at the
National Institute for Medical Research, UK, have recently discovered that the "silent" X
chromosome in females is not entirely silent - some of the genes evade inactivation,
meaning females actually express more genes than their male counterparts.
Approximately, 15% of genes escape inactivation altogether, each of which now becomes
a candidate for explaining some differences between males and females. In addition,
another 10% are sometimes inactivated and sometimes not, giving a mechanism to make
women much more genetically variable than men.
Very early in embryonic development both X-chromosomes are apparently active, but
then most of the duplicate chromosomes are rendered dysfunctional by staying tightly
condensed in the heterochromitin state. It is entirely a matter of chance whether it is the
paternal or maternally derived X chromosome that is inactivated in any given cell, but
once inactivated; all subsequent cells derived from that cell will continue to have the
same inactivated X chromosome. In individuals with visible sex-linked traits the results
can be clearly seen as patches of paternally and maternally derived traits. Thus, all female
mammals are mosaics.
The best known example is the calico cat, which is almost always a female (the
few males are abnormal XXY individuals) displaying a patchwork of black and
orange colors, each patch representing a clone of an original cell that randomly
inactivated either the X chromosome with an allele for orange fur or a the X chromosome
with the allele for black fur. In dogs, we have to look a little more carefully for such
evidence. Examples include X-linked muscular dystrophy in Golden Retrievers and Xlinked hereditary nephritis. Because these female carriers are mosaics for the
abnormalities seen in these diseases, they may exhibit attenuated signs of the disorder,
with the severity depending upon the proportion of the mosaic derived from the X
chromosome that carried the abnormal allele. Sex-linked traits will be passed from dams
to sons via one of her X chromosomes. Because sires have only one X chromosome to
pass on to their daughters, in order for the trait to be fully expressed in a female, she must
have an affected sire and carrier (or affected) dam. The degree of mosaicism that the dam
expresses is random and does not affect the chances of her offspring being affected or the
severity of that trait in those that are.
Misunderstandings about sex-linked inheritance have given rise to many breeding myths,
the most widespread of which place greater emphasis on the “sire line” (sire to grandsire)
in the belief that “what you see is what you get” is due to the single X chromosome, as
well as the belief that important breed attributes are carried on the Y chromosome. Some
also contend that whether an ancestor is on the dam versus the sire’s side of the pedigree
21
is of prime importance. These theories neglect the fact that the Y chromosome contains
few, if any, identified genes apart from those involved with male reproduction, and that
that the sex chromosomes are but one of 39 pairs of chromosomes. These ideas served the
19th century breeder well enough, having been developed well before the birth of genetic
science, but they have no place in the 21st century breeder’s arsenal.
So, the variety of appearance between dogs of different breeds is controlled at several
different levels. Some types of expression seem to depend upon turning
control/regulatory genes on and off so that a specific developmental cascade is expressed.
Other phenotypic differences must rely upon the interaction of genes, their various alleles
and where these hereditary units are located on the chromosomes. Hopefully, the modern
breeder will be able to use this knowledge to make more informed choices when planning
a breeding or to understand why certain breeding decisions went awry.
Chapter 4
ETHICS AND BREEDING STRATEGIES
The canine species as a whole maintains a tremendous genetic diversity. Indeed, it is the
“plastic” nature of the canine genome that has allowed the creation of such a variety of
different dog breeds. By selection for certain behaviors and the physical requirements
needed for a particular occupation, humans were able to fashion breeds as
morphologically different as the Yorkshire Terrier and the Newfoundland. However, in
establishing specific breeds, both breeds and individual dogs have lost genetic diversity.
What does this mean to the fancy?
It is a fact that every dog--and human for that matter--carries deleterious genes. It is
nothing to be ashamed of it is just a fact. Nevertheless, many breeders feel that they must
inbreed or, more euphemistically, linebreed in order to maintain type. However, when
you inbreed, you not only double up on the “good” genes, or those that you are selecting
for, but you are also doubling up on recessive traits that are at the least suboptimal and
which, at the worst, express genetic disease. If the trait is polygenetic, like hip dysplasia,
then you are probably adding to the “threshold” genetic load at which that disease is
expressed. The choices and priorities of individual breeders have a tremendous impact on
the continued viability of their specific dog breeds. The question one should ask oneself
is, are you breeding for yourself and your ego or are you seeking the betterment of, and
indeed the continued existence of, your breed? Let me suggest that some current breeding
practices are neither in the best interest of the individual dog in terms of health and
temperament, nor do they bode well for breeds the future.
BREEDING QUESTIONS
So what defines a breed? It has been suggested by Jeffrey Bragg in an internet article
entitled “Purebred Dog Breeds into the Twenty-First Century—Achieving Genetic Health
for Our Dogs” (www.siriusdog.com/bragg.htm) that three concurrent criteria have to be
met before one can declare with certainty that, yes, this is a distinct breed.
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Dog breeds are first distinguished by ancestry. This means that all the dogs of a certain
breed can trace their pedigree back to a select group known in breeding parlance as
founders or foundation stock. The next condition is that they have been created for an
express purpose, i.e., they all have a specific job to fulfill. Thirdly, they all must share a
particular physical appearance that subsequently has been defined and refined into what
is now known as the breed standard.
Originally, that breed standard may have reflected the type of work required of the dog
but sadly, this may no longer be true. The Bulldog is a good illustration of how
exaggerated type (appearance) has led to the creation of a dog no longer capable of
performing its original function, which was bull-baiting--setting Bulldogs on a tethered
bull with the purpose of pinning and holding it. The breed has lost its agility and now
would get stomped or gored by a bull. The bracycephalic face makes it impossible to take
in enough air to support that vigorous activity and leads to overheating.
The case can be made that rigid selection for appearance and preference given to
breeding partners, based on a closed and genetically isolated population derived from a
particular foundation, has resulted in the loss of genetic diversity and the steadily
declining health of the purebred dog. In addition, little or no emphasis is often placed on
performance factors for which the dog was originally bred. Even though many do not
acknowledge there is a problem, modern breeders are now in a quandary because they
have failed to recognize that techniques needed to establish a breed (such as extreme
inbreeding) are detrimental to the continued existence of that breed.
Dog breeders are a group with long traditions and many “rules of thumb” that are
contrary to known scientific facts. Far too few dog breeders have had any exposure to
basic genetics, much less population genetics, to assist them in making informed mating
decisions. Population genetics involve a population or species as a whole, rather than
concentrating on the individual animal. Population genetics is a very useful tool for
showing what happens when we lose genetic diversity. Our worldwide purebred registries
were developed on presumptions that predate genetics and that do not hold up to
scientific scrutiny today. The idea that inbreeding is not problematic is the prime
example. This must change if we are to save the sport of dog breeding and showing.
When discussing the need for changes, many “old line” breeders argue, “Genetics is just
a science based on theories, and theories have often been proven wrong by newer
theories.” One sees comments on various Internet breed lists alluding that no one
person’s theory has more value than another is because they are only opinions.
Theory and scientific opinion are often misused and misunderstood terms. To a
layperson “theory” means speculation, but in truth, anything in science that gets elevated
to the status of a theory has an overwhelming amount of evidence that supports it and
has, in fact, withstood many challenges. Theories bring together and elucidate a large
chunk of information and help us to understand and organize a wide range of topics.
INBREEDING AND THE PUREBRED DOG
So what has science shown us? In order to create a uniform type which breeds true, one
must inbreed. Inbreeding is the mating of two animals that are more closely related than
23
the average individual within a certain breeding population. When breeds were formed,
usually just a few dogs were used as founders. As a result, many existing breeds are more
than 20 percent ancestrally inbred, as shown in Daniel L. Hartl’s and Andrew G. Clark’s
“Principles of Population Genetics.” 7 To illustrate let us look at the Samoyed, a breed
that started from a foundation stock of fewer than 20 dogs. British explorers had already
taken some of the dogs of the natives local to the Bering Straits for sled dogs in their
quest for the North and South Poles. Taking a small population from a major population
in this way is called a founder event. The British had rather severe selection criteria: The
dogs were to be white, have dark eyes, dark eye rims and solid black lip lines.
Compounding the problem of a limited number of founders was the overuse of
several of these foundation animals, and the underutilization of the others. This artificial
selection was necessary because inbreeding alone is not sufficient to “fix” characteristics
and eliminate unwanted traits. Artificial selection refers to nonassortive mating in which
selection pressures are determined by purely aesthetic factors. For example, a human
deciding he or she likes red coats. So inbreeding and artificial selection were used to fix
type by increasing the homozygosity of the genes that coded for appearance. In addition,
many other traits not expressed in the phenotype also became homozygous. This practice
also resulted in a loss of genetic diversity and the fixing of gene frequency. This means
that the frequency of certain genes found within the source or original population are not
necessarily reflected in the new founder population. It all depends on what genes the
founder animals brought with them. Thus a genetic defect that was very rare in the source
population can become very common in a particular breed because one or more
individuals in the new population carried that defect. Compounding the problem is the
fact that small populations are subject to genetic drift. Genetic drift is the random loss of
alleles due to chance. As explained above, alleles are alternative forms of genes at the
same position on a chromosome. Having multiple alleles at a particular locus within a
population is a measure of that population’s genetic diversity.
One way to illustrate the concept of genetic drift is to think of a coin toss. The probability
is 50:50 that either side will come up. However, if you toss the coin only three times, it is
not all that unusual for you to get three heads or three tails. It is only by increasing the
number of tosses that you start to get the normal probability. This is called the Law of
Large Numbers. Think of the number of tosses as the number of individuals in a
population--the fewer the number of individuals, then the fewer the number of alternative
alleles available. Also adding to the problem is the fact that not every individual is chosen
to produce progeny, so his or her genetic contribution is lost forever. The actual number
of individuals that produce progeny (as opposed to the total number in a population), may
be surprisingly limited in certain breeds. To illustrate, imagine you have a very popular
breed with thousands of registrations yearly. What if only 300 males are used to provide
stud service? Anything that restricts the number of males used will limit the effective
population. This uneven use of individuals in breeding remains a factor today and is
called the popular sire syndrome.
LINE BREEDING AND INBREEDING
Dog breeders have for years expounded on the merits of line breeding as opposed to
inbreeding. Some claim that line breeding has no deleterious effects. This is just not so.
Line breeding is not a recognized term in genetics—it is all considered inbreeding. The
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late geneticist, Dr. John Armstrong of the University of Ottawa, Canada so elegantly
wrote,8
In my view, one could probably subdivide inbreeding into three categories:
background, historic and recent. The background level is dependent upon the
number of founders. In a breed/population that started from six or eight founders,
sometimes closely related, you cannot find individuals that are not related even if
you breed as carefully as possible. Recent (or “close”) inbreeding is, to me, the
breeding of sons to mothers, full siblings, and the like. When it isn’t done simply
for the convenience of the breeder, the usual justification is that it is the only way
to preserve type, or that it is an effective way of discovering problems in your
line. Yes, genetic defects can be uncovered in this way, but in practice I don’t
think many are or they are not recognized as such. Historic inbreeding results
from uneven sampling from the population. This is most obvious with the males.
The same few “popular” (well-promoted) individuals are used repeatedly, and
many of the others are not used at all. The collection of genes from the latter may
be lost to the population, particularly if it is small. Everyone becomes related to
these popular sires and inbreeding becomes inevitable. What appears to happen is
that slightly detrimental genes that individually might not make much of an
impact start to accumulate in the population until breeders begin to notice that
their litter sizes are smaller than they remember the old-timers reporting, they
have difficulty getting a bitch pregnant and that various health problems seem to
be turning up more often than in the past. Some may attribute these problems to
diet, environmental toxins and the like, but the bulk of it is genetic [author’s
emphasis]. This is what inbreeding depression is all about.
COEFFICIENT OF INBREEDING
The coefficient of inbreeding (COI) is the statistical probability that the two alleles at a
randomly chosen gene locus are identical by descent; i.e., inherited from an ancestor
common to both parents. The more inbred the breeding partners are, the more likely that
they will share the same alleles. A common inbreeding paradigm in the dog world is the
breeding of a grandfather to a granddaughter. Although this may be a general concept of
dog breeders, it is contrary to how geneticists, especially population geneticists, think it
should be done. If one ignores any previous inbreeding within the pedigree, the minimum
COI of this breeding is 12.5 percent. Professional breeders of production animals such as
cows, pigs, goats, horses, sheep and chickens, think that a COI around 9 percent is
skirting the allowable limit. They, of course, are interested in such issues as health,
productivity and reproductive viability. One then must ask what dog breeders are
interested in? A COI of 12.5 percent means that it is very likely that the progeny of a
granddaughter/grandfather cross share identical alleles at one out of every eight possible
loci.
The steadily decreasing heterozygousity within the individual breeds is cause for alarm.
Not only is there a loss of reproductive fitness, but other parameters such as longevity
are also affected. A paper titled “Inbreeding and Longevity in the Domestic Dog,” which
was submitted by John Armstrong for publication in the Journal of Heredity, suggests
that in the breeds he looked at, there is a decline in the median life span of about 7
percent for every 10 percent increase in inbreeding. 9 Another example of the deleterious
effects of inbreeding is what is happening to the immune system. More and more we are
25
seeing such problems as autoimmune diseases, irritable bowel syndrome and various food
and environmental allergies. The genes that control the immune system must be
heterozygous if the individual is to have the ability to recognize foreign proteins, to
differentiate foreign proteins from “self” and to fight off disease and parasites without
overreacting to these “normal” environmental perils.
The genes that control the immune system are passed down together as haplotypes, one
set from each parent. They are found so close together on the chromosome that very little
if any recombination occurs. Recombination is the process of combining genotypes and
phenotypes not present in either parent, but which show up in their offspring. When
inbreeding occurs, the chance that a puppy will inherit an identical set of these genes
from each parent increases. This, in effect, cuts the functional ability of the immune
system in half and seriously compromises the quality and duration of life for the puppy.
Those of you who have had a dog with allergies, with demodectic mange or without the
ability to fight off a deadly disease know the tremendous suffering this involves, both for
the dog and its owner.
There are other reasons for an impaired immune response, such as poor nutrition or a lack
of vitamin E and selenium in the dam’s diet (Fed Proc. 1979 Jun;38(7):2139-43.
Influence of vitamin E and selenium on immune response mechanisms.
Sheffy BE, Schultz RD.)
Without those two nutrients the offspring are born without a sufficient number of
immune competent cells. So there are environmental reasons for an impaired immune
system, but the bulk of the literature suggests that inbreeding plays the greater role.
THE RAMPANT RABBIT
Arguments against inbreeding are controversial in the dog fancy. Questions similar to the
following are often asked: “Wild rabbits arrived in Australia in 1859, when Thomas
Austin released 24 animals he had brought from England for sport hunting; why didn’t
the rabbit inbreed itself to death?”
This story is a good illustration of the problems associated with dog breeding. The
first difference between dogs and rabbits is that the rabbits were not being selected for
anything other than survival. They had the additional advantage of having an almost
unlimited food supply, no effective predators and really no competition for their
particular ecological niche. In fact, there was no natural selection to begin with because
few if any diseases and parasites came with them. The breeding was as random as
possible. The original rabbits had lots and lots of offspring, who also bred randomly, so
the founder’s alleles were comparatively evenly distributed during the first explosive
phase of population growth.
Once the rabbit population was large enough to meet the Hardy-Weinberg criteria of
about 10,000 to 100,000, the gene pool was pretty safe from genetic drift. The HardyWeinberg criteria states that the population needs to reach a certain number of individuals
for it not to be subject to genetic drift. 10 Considering that rabbits breed like, well,
rabbits, they undoubtedly reached that population cushion fairly rapidly. On the other
hand, certain dog breeds were intensively selectively bred right from the first generation
and for criteria that had nothing to do with survival: In the Samoyed it was all-white
26
coats, black lip lines and prick ears; thus, breeding was by no means random. In addition,
because the population was never large enough early in the breed history to protect these
dogs from genetic drift, the random loss of alleles was a serious problem. Loss of alleles
that code for big brown spots does not matter in this breed, but what about those that
control the immune response or allow an individual to metabolize an environmental
toxin?
There are alternatives to inbreeding. Assortative mating is the selection for
breeding of phenotypically similar individuals. For dog breeders this means that when
choosing a mate for a bitch, you find a male that matches all the physical appearances or
traits within the breed standard that you want to keep and that do not duplicate any of
your bitch’s faults.
Selection by phenotype is very common in those European countries where inbreeding is
discouraged. According to M.W. Willis in Genetics of the Dog, most German breeds are
bred with very little inbreeding--instead they use assortative mating and selection. 11 This
results in a very uniform type among dogs appearing in the show ring. Assortative mating
does increase the resemblance among littermates; however the difference between the
two breeding techniques is that the chance of doubling up on hidden or undesired traits is
minimized with assortative mating, even though the breeder is selecting the animals. This
is not true of inbreeding.
PRESERVING GENETIC DIVERSITY
The optimal program for promoting genetic health involves breeders using assortative
mating and avoiding inbreeding as much as possible in order to minimize the coefficient
of inbreeding. Open the studbooks, and, if possible, use the original stock. This is just
what was attempted with the Basenji. In 1987, two Americans, Jon Curby and Mike
Work, embarked on a mission for their breed that would take them to Northeastern Zaire
on a search for Basenjis in their native land. One by one they located dogs and bartered
with their sometimes reluctant tribal owners, until seven African-born Basenjis were
loaded for the trip home.
In order for their mission to be a success, however, the dogs had to be admitted into the
studbook. The impetus for their mission was the existence of certain genetic health
problems in the AKC Basenji that had become widespread due to the breed’s limited
gene pool. If these dogs could contribute their genome, it might be the answer to the
breed’s problems, but without pedigrees, little chance for AKC registration existed. Their
best chance lay in a roundabout route, by trying to register the dogs with a foreign kennel
club with less strict pedigree requirements, and then trying to register the dogs’
descendents (after three generations) with the AKC. No guarantees existed as to whether
the scheme would really work. It appeared the ability of these dogs to contribute to the
AKC gene pool was uncertain, and at best, years in the future. In the end, perhaps they
took the least likely yet most logical approach: A direct appeal to the AKC. The AKC
approved the opening of the studbook to imports from this and a subsequent expedition,
pending the approval of the Basenji Club of America, which was given in 1988.
Yet many questions remained. How would they deal with the fact that most of the
imports were brindle patterned, a pattern not even allowed by their AKC standard? How
27
would these dogs straight from the bush be accepted in American breeding programs?
Would the influx of their genes sully their descendants and make them less competitive in
the show ring? Recognizing that brindle was actually the most prevalent pattern in its
native Africa, the Basenji Club of America amended its standard to allow brindle
patterning, and many breeders soon considered their brindle Basenji’s coloration a badge
of pride. There was no compromise of quality--the top show Basenji, in fact the top
hound in America for 1997, was a brindle Basenji. Most important of all, the new Basenji
genes have so far proven to be free of the hereditary problems that had beset the breed.
Another way to maintain genetic diversity is to allow breeding between different strains
of dogs that are really the same breed but that have had artificial breed status conferred
upon them by the various registries. There have been numerous artificial breed splits
along color lines or sizes or based on politics. One bitter dispute is between the American
Kennel Club’s Akita vs. the Federation Cynologique Internationale’s Great Japanese Dog
vs. the Japanese Kennel Club’s Akita. There were never very many Akitas in Japan.
Fewer still survived World War II. After its recognition of the Akita, the AKC closed the
studbook on Akitas from Japan, effectively cutting the genetic pool of Akitas off from
their land of origin.
The politics innate in the registries have not followed rational genetic lines but rather
have been subject to the demands of power, influence and winning kennels. Basenji and
Saluki breeders understand firsthand what we mean by politically restricted gene pools
subordinated to a European concept of purebred dogs. These two breeds of great antiquity
are not AKC-recognized unless they come from just a few founders. It matters not that
they have been around for several thousands of years and are still numerous in their lands
of origin.
HIDING GENETIC DISEASE
If breeders withhold information about the genetic disease in their breed or within their
line, then we face an insurmountable barrier in any attempt to control those diseases.
Open discussion about problems your dogs have produced allows other breeders to make
more informed choices. Secrecy and denial only perpetuate the problem. Genetic testing
may help; however, if the disease does not appear until late in the dog’s life, then only by
alerting your puppy owners “downstream” from the affected dog can you hope to prevent
further misery for both the dogs and owners.
False pedigrees, absent genetic testing, can invalidate the conclusions drawn from
pedigree analysis. We recognize that there is some “noise” (false information) in the
various registries and, in some cases, a significant level of noise. This makes pedigree
analysis difficult and in some cases impossible. The SCC (French Kennel Club) has done
random paternity and maternity checks on about 200 pups from recent litters from
various breeds. The parents of 17 percent of the pups as indicated by the pedigree were
incorrect. We suspect that this French example is not only a French example, but a
worldwide example. This may occur even more often in the United States where there is
a significant amount of money changing hands between commercial breeders and pet
stores. This quote from C.A. Sharp, author of “The Biggest Problem,” in the Summer
2000 edition of Double Helix Network News, says it all succinctly:12
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You all know them. The ones that put winning above all other goals. “It doesn’t
matter as long as the dog wins,” is their mantra. Their dogs must win, as must
their dogs’ offspring, and woe betide anyone who stands in their way as they
pursue greater breed and personal glory . . . If a genetic problem isn’t apparent
they will ignore it. If it can be (surgically) fixed they will. If it can’t, they will
employ some variant on “shoot, shovel and shut-up,” or recoup their losses by
shipping the dog a long ways away, preferably across an ocean or two. If someone
else knows about the problem, the Incorrigibles will use any means at their
disposal to shut that person up, ranging from veiled threats and rumor-mongering
to blatant bully tactics and threatened legal action.
Most of us can think of an example of this behavior. Amongst Samoyeds it was the attack
on Rosemary Jones, the breeder who first brought the dirty little secret of progressive
retinal atrophy into the light of day and who named names and published pedigrees.
Without acknowledging there is a problem, how can we fix it? Why is it also that we
speak among ourselves about these unethical breeders and yet we do business with them
because . . . their dogs win! What does this say about our own ethics? The form of PRA
expressed in Siberian Huskies and Samoyeds is an X-linked, late-onset disease that
usually appears somewhere between 3 and 5 years of age. By testing breeding stock,
breeders will be able to avoid producing affected offspring. Research on the disease was
done at the James Baker Institute, Cornell University and was funded by a combined
grant from the AKC Canine Health Foundation and the Siberian Husky Club of America.
The test is offered by Optigen®, LLC (www.optigen.com).
Let’s move on to the Ostrich Syndrome breeders. These are those among us who
will do anything not to test for a genetic disease. If they do not test, they will never find
it. Denial is the name of that game. Those of us who are truly dedicated to the health of
our canine companions will not make any headway until we first recognize and confront
the human behavior expressed when faced with canine genetic disease. One can conclude
that the genetic problems in purebred dogs are not intrinsically a canine problem, but
rather a human problem supported by politics, old wives’ tales, ignorance and even
outright rejection of scientific opinion.
To conclude, Basenjis are far from being the only breed to suffer from genetic health
problems caused in great part by small gene pools. Purebred dogs have gained the
reputation as genetic disasters. Many breeds count fewer than 50 dogs as their foundation
stock—a number that would send shudders up the spine of any wildlife biologist seeking
to save an endangered species. Breeding schemes for endangered species focus on
increasing the gene pool as much as possible by integrating all available animals. In
many of our breeds of dogs, the gene pool is out there—we just can’t jump in because of
the policies of our registries and clubs.
Chapter 5
THE SHALLOW END OF THE GENE POOL
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The ability to go back to stock from a breed’s country of origin (COO) in order to expand
the gene pool is a process known as introgression. The basic tenants of the AKC make
such a process difficult to impossible for dogs originating in most “non-western”
societies unless special exceptions are made. Many COO dogs come from countries in
which registries do not exist or do not meet the AKC’s specific standards. In the early
development of many breeds, the AKC often facilitated expansion of the gene pool. Most
recent attempts to introduce new genetic material by the registration of COO stock have
met with the AKC’s steadfast position that unregistered stock cannot be directly
integrated into the studbook. The AKC will sometimes make exceptions in the face of
compelling health or medical reasons. In such cases the parent breed club must vote in
favor of such a step and then petition the AKC to open the studbook for a brief time.
The Saluki, for example, is an ancient breed that still exists throughout its native
Middle East. Its Bedouin owners can recite its pedigree for generations, but such is not
acceptable proof of purity for the purposes of AKC registration. The AKC Saluki is
derived in most part from a small number of founding dogs brought to England around
the 1920s. In 1945, two Salukis bred by King Ibn Saud came to this country and after
some persuasion, the AKC made a special ruling whereby descendants removed by three
generations from these imports could be registered, as long as the generations in between
were always bred to registered stock. Today, imported COO Salukis are formidable
competitors in coursing trials. Attempts to gain AKC recognition for these dogs,
however, have been unsuccessful, largely because of a lack of consensus by the parent
club as well as the lack of any overwhelming genetic problems that would lend urgency
to the matter.
IN THE CROSS FIRE
In some breeds, no COO stock exists, or that which does exist shares the same problems
as the AKC stock. In such cases, crosses to other breeds may be the only way to
introduce new genes. Early in the creation of breeds, such crosses were commonplace.
For example, although the Shih Tzu is an ancient breed, at the beginning of this century
the breed is thought to have become extinct in China. Modern Shih Tzu descend from
seven dogs and seven bitches, one of which was not a Shih Tzu, but a Pekingese. This
cross occurred in 1952, long before AKC recognition of the breed. While the early
registration bodies sometimes sanctioned crosses to other breeds, after a breed was
established, they allowed crosses only in the rarest of circumstances.
The Dalmatian is a breed with a genetic predisposition for abnormal uric acid metabolism
that leads to painful and debilitating stone formation. In 1988, at the behest of the board
of directors of the Dalmatian parent club, and with the approval of the AKC, a cross was
made to a Pointer in an attempt to introduce the genes for normal uric acid metabolism
into the Dalmatian genome. The plan was to breed the normal progeny of this initial cross
back to Dalmatians, continuing for many generations until their descendents were
essentially Dalmatians with no trace of Pointer (except for the normal uric acid
metabolism). With each backcross (crossing the mixed progeny back to pure
Dalmatians,) the proportion of Pointer chromosomes would decrease by one half. This is
a common plan for the introduction of a new gene into another population, although
several factors can slow or halt its progress. These factors include linkage wherein the
30
trait being selected for is on the same chromosome as other traits that may be essential for
type. If, for example, the trait for normal uric acid metabolism was on the same
chromosome as the trait for patches, acceptable in Pointers but not in Dals, a decision for
health would also be a decision against type. Even so, in time, the Pointer derived
chromosome bearing the introduced allele will cross over and exchange genetic material
with its homologous Dalmatian derived chromosome and hopefully breed true for the
Dalmatian type without the metabolic defect.
This did not happen, though. The descendants with normal uric acid metabolism tended
to have ticking, instead of spotting, suggesting the possibility of either linkage or
pleiotropic effects. Rember that pleiotropic effects are those where one gene causes
several diverse effects. Further problems arose with a change in consensus about the
project within the Dalmatian Club of America. The club subsequently objected to the
registration of the crossbred progeny and lifted the registration privileges for these dogs.
Thus, although the experiment was a medical success, it was not successful from the
viewpoint of maintaining Dalmatian type or achieving widespread acceptance. The
important lesson in this case, however, is not that the venture failed, but that the AKC
had the foresight to approve it in the first place. Perhaps the most important lesson was
one of requiring full consensus of all parent club members before undertaking a project of
this nature--“full consensus” implies unanimous, which is impossible in most clubs. The
AKC now requires a full membership vote from the parent club before granting approval.
One of the rarest breeds in America is the Wirehaired Pointing Griffon. In the 1980's, the
breed’s limited gene pool resulted in the decision by some breeders to cross the breed
with the Cesky Fousek, a European breed. Griffon breeders did not universally approve
the project and the AKC did not grant recognition to the resultant dogs. Breeders have the
liberty of choosing the direction of their own breeding programs. If they choose to cross
their dogs to another breed without a priori parent club and AKC approval, they cannot
expect AKC recognition of their stock--no matter how good their intentions.
While it is clear that in some cases the AKC will consider “breaking the rules” in order to
promote genetic health and diversity, no set guidelines seem to exist by which a parent
club can petition for such an exception. Shouldn’t a published set of criteria be available
so that breed clubs know at what point they may reasonably resort to this step? Should
the breed club or the AKC be the final authority when deciding if such exceptions are to
be made?
It is the parent breed clubs that initially develop breed standards, maintain stud
books, and petition AKC for recognition of the breed and the club. Even after agreeing
upon a standard, the parent club may consider amendments to it and, if deemed
appropriate, change it. In developing a standard, a parent club exerts tremendous power
over the genetic future of a breed. In some cases, standards require physical
characteristics that are inconsistent with hardiness. For example, brachycephalic features
predispose a dog to breathing difficulties, diamond shaped eyes to entropion and/or
ectropion, excessive wrinkling to moist dermatitis, and excessive size coupled with deep
chests to gastric torsion (bloat). In most of these cases, the breed standards were drawn
up long before the association of the traits with physical difficulties was known. Such
traits have become so ingrained as basic to breed type that breeders and parent clubs
31
choose to retain them despite their associated problems. Since the parent club has sole
discretion over the breed standard, only the breed club can effect a change in the standard
to change the essence of type and reward healthier, but less traditionally typey,
specimens. In almost every case in which type has been at odds with health, parent clubs
have chosen to give type precedence. The results are obvious!
Breed standard disqualifying faults also affect the genetic health of a breed. The AKC has
several disqualifying faults applicable to all breeds; perhaps the best known of them is
unilateral or bilateral cryptorchidism--the failure of one or both of the testicles to descend
normally into the scrotum. Since this fault is less detrimental to health than a plethora of
other far more serious faults with far greater heritability, the universal disqualification of
such dogs is of questionable value to any breed. Several parent clubs impose further
disqualifications, usually for traits considered extremely untypical for the breed.
Common disqualifying faults are for dogs over or under a certain weight or height, for
different eye color, or coat colors or types. Dogs with disqualifying traits cannot be
judged at a conformation show, but may compete in other venues. By banning these dogs
from conformation competition, parent clubs hope to discourage breeding from them and
perpetuating the offensive trait. Removal of dogs from the breeding population based
upon arbitrary aesthetics can do more harm than help, especially in cases where the breed
has a limited gene pool and the banned trait has no strong hereditary component.
Lack of appreciation of genetic aspects of a trait can result in illogical and detrimental
disqualifications. One example is the Mantle (also know in the USA as the “Boston”)
Great Dane. These dogs are black with typical “Irish marked” coat pattern, that is: white
feet, tail tip, muzzle, and collar, just like the typical pattern of the Boston Terrier. This
color pattern was, until recently, listed as a disqualifying fault under the AKC standard
for the breed. Yet serious breeders of Harlequin Great Danes have routinely used the
Mantle in their breeding programs. Breeding a Harlequin to another Harlequin
statistically results in 25% Harlequins, 25% merle (disqualified), 25% white (disqualified
and commonly defective) and 25% Mantle (until recently also disqualified). This gives a
predicted percentage of show marks of no more than 50% (some further losses may occur
from unsuitable markings on Harlequins and Mantles). Breeding a Harlequin to a Mantle,
however, results on average in 25% Harlequin, 50% Mantle, and 25% merle; with
potentially 75% show marks produced in this breeding. Thus, a substantial proportion of
dogs produced from the traditional breeding of Harlequin to Harlequin--perfectly
acceptable colored parents--will be disqualified from breed competition by virtue of the
combination of acceptable genes that together produce an unacceptable color pattern in
this breed. Some of these disqualified dogs may be of such high quality otherwise that
they are sought after for breeding back to Harlequins.
Before the introduction of the Mantle this was often the case as their use as a breeding
partner to a Harlequin, as noted above, actually results in a greater percentage of
acceptably colored offspring than would a Harlequin to Harlequin breeding. It also avoids
the production of potentially defective homozygous merle dogs, known as white Danes.
Unfortunately, because these Mantles were disqualified from competition, their quality,
until recently, could not be objectively judged by way of conformation awards or titles. It
was obvious to many that the standard with these disqualifications was
counterproductive. Finally, in 1996, in recognition of the importance of the Mantle Dane
32
to the breeding of Harlequins, the Great Dane Club of America voted to change the breed
standard to accept the Mantle Dane as an acceptable color. Changing the standard is one
of the heaviest responsibilities that a parent club can undertake, and to do so in
recognition of genetic mechanisms is a progressive step for a parent club. Unfortunately,
not all clubs have shown such an ability to accept genetics over tradition.
In other breeds, disqualifications have been implemented in recognition of health
problems related to certain traits. In 1979, a “white” Doberman Pinscher named Sheba
was AKC registered. She was undeniably eye-catching, with a light cream coat,
translucent blue eyes, and pink nose and eye rims. Her offspring were crossed to each
and produced more such dogs. These striking animals aroused much interest, but were
apparently tyrosinase positive albinos. Not only were these dogs considered untypical for
the breed, but because albinism can be associated with health problems, especially those
from ultraviolet exposure, the Doberman Pinscher Club of America acted not only to
disqualify these dogs, but worked with the AKC to develop a scheme whereby dogs
possibly carrying the gene for albinism could be identified by their registration numbers.
Such dogs are identified with a “Z” as part of the litter or individual registration, the Club
having acted proactively to limit this problem—a very postive move.
In recent years, most parent clubs have formed breed health committees, the success of
which depends upon many factors. Some clubs have a large membership from which to
draw and an open door policy which has brought in a diversity of educated and dedicated
committee members. But some clubs still operate under closed memberships, in which
prospective members must be sponsored by existing members and voted upon by the full
membership. These clubs have been less apt to deal with health issues while preferring to
preserve the status quo. Political issues, of course, are rampant within any breed, and
control of the parent club means control of the breed standard--and ultimately the future
of the breed. Thus, in certain breed clubs, because of either small breed numbers or
exclusionary practices, the chance of forming a strong health committee is considerably
lower than in those clubs with a more diverse membership.
The first step a breed health committee faces is identification of health problems. This
step is not as simple as it may seem. Breeders may have a “feeling” about what may be a
problem based upon personal experience and anecdotal reports. The problem then
becomes one of determining whether these problems are breed specific or common to all
breeds. For example, if a breeder knows of ten dogs over the past three years that
suddenly fell over dead at a young age, this might raise some suspicion that the breed had
a problem. But perhaps this is no more than would be seen in any breed of dog. The
problem is that 95% of that breeder’s contacts also have the same breed of dog; it would
be very unlikely that the breeder would ever hear about the same circumstance in another
breed simply because of lack of communication. Thus, a major problem in breed specific
health surveys is one of bias.
While it is unrealistic to expect parent clubs to have the expertise conduct statistically
sound and unbiased health surveys, they are being forced to shoulder this responsibility.
Some have done a better job of tackling it than others. The greatest barrier to parent club
health surveys is lack of trust on the part of breeders, since those collecting the
information are often that breeder’s competitors. Though hiding health information may
33
seem petty and dishonest, recall that many breeders have a lifetime of hard work, study,
money, and emotion invested in their line of dogs. They fear that if they are the only ones
to come forward with information, they may be the only ones branded as having
unhealthy dogs, effectively terminating the line to which they have devoted their lives. In
popular breeds much of the information thus comes from individual pet owners. In some
other breeds efforts are undertaken to ensure anonymity.
For example, the Salukis In Good Health Committee developed a process in which
identifying information and medical information pertaining to a dog are sent in separate
sealed envelopes, coded by a “middle-man”, and sent on to separate data entry people so
that no person ever sees the medical and identifying information together. Only in the
final step are the two sets of information associated within a third database that encrypts
the information so that actual identification of animals is still inaccessible to committee
members. It is this information that is ultimately used for performing analyses.
CODE OF ETHICS
Most parent breed clubs maintain a standing ethics committee to develop, maintain and
enforce some form of code of ethics. Such codes are known by various names such as
Guidelines for Responsible Ownership, Guidelines for Breeders, Guidelines for Ethical
Conduct, Ethical Guidelines, Mandatory Practices, Principles of Integrity, Statement of
Conduct, Canon of Ethics, Breeders Code and Code of Recommended Practices. There
are several more variations upon this theme, but in general, the parent clubs’ codes of
ethics make vague and not very binding statements about genetic health, ranging from no
mention at all, to actually listing the diseases of interest and the screening required.
Some parent clubs have had serious internal political problems when establishing a
standing ethics committee, with the result being that some clubs have yet to progress this
far. Other clubs have official committees, but they are kept out of sight and out of mind.
In some clubs, because of the personalities and beliefs of some of the more successful
members, those individuals have severely controlled or thwarted the actions of such
committees. When one of the more successful breeders with more champions bred and
shown refuses to screen for hip dysplasia, the club is often powerless to enforce screening
requirements. In such cases, genetic screening becomes “recommended,” “encouraged”
or “should be considered.”
There are several breed clubs that do in fact list the screening that should be done and say
that such screening is mandatory under the code of ethics. A number of codes of ethics
mention the Orthopedic Foundation for Animals (OFA), but no other registry such as
the Canine Eye Registration Foundation (CERF). This is a step in the right direction,
but one finds virtually no evidence that anything has been done to discipline or terminate
membership of any successful breeder who fails to follow the club’s screening regimen.
In fairness to breed clubs, it is difficult in a litigious society to attempt to force any sort of
genetic testing without that attempt resulting in internal lawsuits. If changing a breed
standard to add a comma to correct punctuation is difficult, think of how difficult it might
be to establish a genetics committee to interface with an ethics committee and develop a
genetic screening regimen appropriate to the breed. Imagine the legal tests when a
successful breeder is censored by some means for not conducting such screening.
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The answer lies in educating the club membership. Advertisements of top dogs need to
include the genetic screening supporting them. Articles need to be frequently published in
the club and breed magazines/newsletters questioning the folly of purchasing a dog from
anyone that did not have an effective genetic screening regimen. In our democracy, the
free market exerts the force for change that is otherwise prevented by the costs of
litigation. The puppy buying public is slowly becoming aware of the problems of
genetically inferior dogs. States are rushing to enact puppy lemon laws.
To some, the AKC is becoming known as the registry of sick dogs. Any breed club’s
attempt to rise above the mire will serve to differentiate that breed from the “You don’t
want one of those, they have a lot of health problems.” Individual breeders can enhance
the desirability of their puppies by documenting generations of genetically healthy dogs.
SAMPLE CODES OF ETHICS
Following are representative extracts from a sample of various breed clubs codes of
ethics:
Akita “I will keep well informed in the field of genetics and work to eliminate hereditary
defects from the breed….I will participate in a program of hip x-raying and eye
examinations by qualified veterinarians to eliminate hip dysplasia and congenital eye
problems. When an Akita has hereditary faults of such nature as to make his or her use
for breeding detrimental to the furtherance of the breed, that dog shall be
neutered/spayed.”
Basenji “Ethical breeders should discuss openly and honestly the genetic and
physical problems that have occurred in their lines. This should include the
potential of these problems to be passed on, especially in cases where testing can
indicate only that a dog is currently free of a problem, but cannot determine that the
problem or the ability to pass it on will not be inherited. Stud dogs or brood bitches who
produce offspring of consistently poor quality or with genetic problems known to be
inherited in the breed are therefore of no value as breeding stock and should not be used
again.”
Basset Hounds “Breedings will be directed toward producing Basset Hounds of
exceptional quality in breed temperament, Basset Hound type and ability to hunt game.
Only healthy and mature dogs and bitches free of congenital defects and of characteristic
breed type, sound structure and temperament shall be bred.”
Borzoi “No animal selected for breeding should have any serious hereditary defects as
determined visually and by veterinary examination.”
Chesapeake Bay Retriever “Be aware of genetic defects which can be harmful to the
breed. When breeding, endeavor to select animals that will reduce the incidence of
genetic problems while enhancing the positive attributes and abilities of the breed. Be
open with all persons interested in the welfare of the Chesapeake Bay Retriever and
discuss possible physical or temperament defects in your own stock.”
Dachshund no statement concerning genetic fitness for breeding.
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Doberman Pinscher “Stud dogs should not be bred prior to one (1) year of age and
should be in good health and free from communicable diseases and disqualifying faults.”
...Any bitch accepted for stud service should be at least 18 months of age, in good health
and free from communicable diseases and disqualifying faults.”
English Cocker Spaniels no standing ethics committee. Statement of Conduct is silent
on genetic health.
Field Spaniels “Breed only healthy and mature animals who are free from serious
congenital and hereditary defects.”
Golden Retrievers “Owners of breeding animals shall provide appropriate
documentation to all concerned regarding the health of dogs involved in a breeding or
sale, including reports of examinations such as those applying to hips and eyes. If any
such examinations have not been performed on a dog, this should be stated.”
“Animals selected for breeding should:
(i) be of temperament typical of the Golden Retriever breed; stable, friendly,
trainable, and willing to work. Temperament is of utmost importance to the breed and
must never be neglected;
(ii) be in good health, including freedom from communicable disease;
(iii) possess the following examination reports in order to verify status concerning
possible hip dysplasia, hereditary eye or cardiovascular disease.
Hips: appropriate report from Orthopedic Foundation for Animals; PennHip; Ontario
Veterinary College; BVA/KC Hip Score (Great Britain) or at least a written report from a
board-certified veterinary radiologist (Diplomat of the American College of Veterinary
Radiologists).
Eyes: appropriate report from a Diplomat of the American College of Veterinary
Ophthalmology (ACVO), or from a BVA/KC approved ophthalmologist (Great Britain).
Hearts: appropriate report from a Diplomat of the American College of Veterinary
Medicine, Cardiology Specialty.
Consideration should be given also to other disorders that may have a genetic component,
including, but not limited to epilepsy, hypothyroidism, skin disorders (allergies), and
orthopedic disorders such as elbow dysplasia and steochondritis.”
German Shepherd Dog no statement of genetic health in the Breeders’ Guide.
German Wirehaired Pointer “Only those dogs free of recognized genetic defects shall
be used in a breeding program.”
Italian Greyhound “It is not always possible to prevent the occurrence of inherited
diseases, as there are not yet definitive tests to identify carriers of genetic diseases. A
breeder's obligation with regard to genetic diseases is to make every effort to prevent
their occurrence and to share openly and honestly all information available regarding the
genetic health status of his/her dogs. While elimination of genetic diseases is a worthy
36
goal, the converse is that excessive culling of animals from the gene pool may have the
equally deleterious effect of limiting the gene pool in the breed. Breeders should be
cautious about removing animals from the breeding pool solely because they are distantly
related to an affected individual.”
Irish Setter “Make every effort to learn about the structure, anatomy, action, behavior
and other inheritable traits of the Irish Setter. To use this information to adhere to the
breed standard and produce sound, healthy dogs with good temperament…To use or give
service only to registered stock that is believed to be free of serious abnormalities which
are considered inheritable….When selling an Irish Setter known to manifest hereditary
defects considered to be detrimental to the breed, use written contracts or spay/neuter
agreements to prevent the dog from being bred.”
Miniature Pinscher “Breed only mature animals in good health, free from
communicable diseases and major genetic faults.”
Pekingese no mention of genetic health in Code of Ethics.
Pointer “Only animals of quality with characteristic type, sound structure and
temperament, and free of congenital faults should be bred.”
Pugs no mention of genetic health in Code of Ethics.
Rhodesian Ridgebacks “Only dogs screened and certified clear of hip dysplasia shall be
bred. Breeders are encouraged to screen for all appropriate hereditary disorders.”
Rottweilers “Breed only AKC registered dogs and bitches which have OFA certified
hips (or HD-free hips as certified by foreign counterparts of the OFA). Imported
Rottweilers must have OFA hip certification within six months after arrival in U.S.A. If
semen is used from an imported Rottweiler, the dog must be x-rayed and certified by the
OFA or foreign counterpart at no less than 24 months of age. Breed only dogs and bitches
of stable temperament with no disqualifying physical faults according to the AKC
Rottweiler Standard (i.e. entropion, ectropion, overshot, undershot, wry mouth, two or
more missing teeth, unilateral cryptorchid or cryptorchid males, long coat, any base color
other than black, absence of all markings.) Offer at stud with a signed written contract,
only mature (two years of age or older) healthy dogs with OFA certified normal hips, free
of communicable diseases, having none of the faults listed in Section 2 above. Refuse
stud service to any bitch not meeting the same requirements. Breed only bitches two
years of age or older with OFA certified normal hips, in good health, free of
communicable diseases, having none of the faults listed above in Section 2, to not more
than one stud dog at any one season, and not more than two out of three consecutive
seasons. Plan all litters with the goal of improving the breed.”
Saluki “To protect every Saluki from the suffering of genetic diseases, affected
individuals will not be bred from.”
Samoyed “Each litter is the result of conscientious planning, including consideration of
the parents’ freedom from hereditary defects, type, soundness, temperament and general
37
conformance to the official standard of the breed. The SCA member must be particularly
concerned with the proper placement of puppies, both pet and show potential. The SCA
member only breeds healthy, mature Samoyed adults, preferable 24 months of age, but at
least 18 months of age. Prior to breeding any Samoyed, the SCA member obtains
certification that its hips are normal from the Orthopedic Foundation for Animals, an
equivalent foreign registry, or from a board approved radiologist and has its eyes certified
free from genetically transmitted defects by a certified Veterinary Ophthalmologist. The
SCA member knowingly breeds Samoyeds only to other registered Samoyeds.”
Scottish Deerhounds “Breeders are urged to breed only dogs and bitches that are in
good health and of such maturity (yet not past their prime) to demonstrate a degree of
freedom from genetic defects breeders are urged to test for health defects, where
possible.”
Shih Tzu “In my breeding program I will keep alert for and work to control and/or
eradicate inherited problems and conditions that are particular to my breed, and breed as
closely to the standard of the breed.”
Silky Terriers “All breeding stock should be of sound temperament, free from
congenital defects such as blindness, deafness and dysplasia. Dysplasia of the hips and
shoulders may be ascertained by x-rays taken and read by a veterinarian who is familiar
with the proper procedure and diagnosis.”
Visla “Breed only those dogs who are free of serious hereditary defects including
epilepsy, progressive retinal atrophy, von Willebrands, entropian and cranial muscular
atrophy and who are over two years of age and have been xrayed and OFA certified as
free from hip dysplasia.”
Weimeraner “Choose only healthy parents of good temperament and qualities in
relation to the Weimaraner ‘s AKC-approved official standard, and whose hips have been
X-rayed and certified free from hip dysplasia by either the Orthopedic Foundation for
Animals (OFA) or any ABVR certified veterinarian. Not use dogs with hereditary defects
or disqualifying faults for breeding.”
Yorkshire Terriers “Prior to breeding, owners of stud dogs and bitches will adequately
screen for both infectious and hereditary diseases, using current techniques as well as
those developed in the future.”
MONEY FOR RESEARCH
The development of genetic tests is an expensive and time-consuming process. Often the
same disease in two distinct breeds is the result of a different mutation. This requires a
separate test for each breed. With the advent of the AKC Canine Health Foundation
(include a URL here), individual clubs are able to raise money for genetic research and
have that money matched by grants from the foundation. Other benefits of using the
AKC/CHF are: Their ability to screen and evaluate research proposals, locate qualified
research facilities, supervise and assess on-going research projects, and prevent the
duplication of management and administrative functions, thus saving time and money.
38
Even more important than money is the raw material needed to conduct the research. This
is where the individual breeders and breed clubs can make a most necessary and
invaluable contribution. Without blood or cheek swab DNA samples, accompanied by
accurate and appropriate pedigrees, genetic research cannot continue to advance. With
this information, tests can be developed so that breeders will have the tools to make
informed and responsible breeding decisions, and rectify some of the extensive health
problems our dogs suffer.
It is strongly suggested that breed clubs look at the heritable diseases associated with
their breeds, and establish a well-defined screening protocol mandatory for all dogs
owned or bred by members of the club. The AKC Canine Health Foundation is there to
help you. Furthermore, it is recommended that the code of ethics include a statement to
the effect: “Members, when advertising any dog, bitch or puppy, in any venue, will
include in that advertisement the genetic screening conducted on that animal and its
parents.” Such mandates are within the prerogative of breed clubs, and only they have the
power to correct the current appalling situation of poor genetic health. It is time to stop
bashing the AKC—“we are them and they are us.” The responsibility for requiring
genetic screening rests squarely with the parent clubs.
Chapter 6
CANCER, IMMUNE PROBLEMS AND
VACCINATIONS
According to the AKC figures, the incidence of cancer in the purebred dog is epidemic.
Why is this so? Due to advances in veterinary healthcare, many dogs are living to an age
where cancer is more likely to appear. We are also living in a polluted environment. Our
canine companions are at an even higher risk for exposure to environmental toxins. Not
only do some of us load our dogs up with flea and tick collars and dips, but their
grooming habits make it much more likely they will ingest pesticides and other chemical
carcinogens. One ubiquitous carcinogen is found in the outgassing of asphalt on hot days.
It is also seen in meat that has been charcoal broiled. The chemical name for this
substance is benzo [o] pyrene, and dogs simply crossing the street can get it on their paws
and later lick it off. This chemical does its dirty work by causing missense mutations, a
type of mutation that causes the replacement of a different amino acid in a protein and
that can result in cancer.
At a recent conference, hosted in part by the AKC, it was revealed that cancer is the
leading cause of death in dogs, after euthanasia. Lymphomas are the most common
cancer found in canines, comprising about 20% of all malignancies. In humans, this type
of cancer has been associated with chromosomal anomalies, it is most likely that this will
prove to be true for dogs as well. What must be emphasized is that all cancers have a
genetic component. We know that there are familial and breed related cancers and that
only emphasizes the genetic aspects of the disease. Identification of affected families
within the canine population may lead to the discovery of cancer susceptibility genes. It
39
is no surprise to learn that those breeds with very small foundation numbers and those
breeds with an overabundance of popular sires are those most valuable for this study.
THE GENETICS OF CANCER
Two classes of genes are suspected of being involved in the occurrence of cancer
when they are mutated: Tumor-suppressing genes and proto-oncogenes, genes that
function to encourage and promote normal growth and division of cells. The progression
of tumor growth correlates with mutations that activate oncogenes (mutated protooncogenes) and render tumor-suppressor genes inactive. These mutations somehow
“uncouple” the same mechanisms that allow normal cell division. What is so frustrating
for both researchers and clinicians alike is that different combinations of mutations are
found in different types of cancer and even in cancers of supposedly the same type in
different patients. This reflects the random nature of these mutations.
Cancers caused by these loss-of-function mutations are more likely to be inherited.
Parents that pass on a mutation in one copy of the gene produce offspring with a
predisposition for cancer in that the disease requires only one mutation in the remaining
“good” copy of the gene to be expressed.
Another familial type of cancer predisposition would be those that involve DNA
repair. The body has mechanisms in place to detect errors in duplicated DNA. If
mutations occur in the genes that code for the proteins responsible for this repair process,
bad copies of the cellular DNA will accumulate. The initiation of cancer requires multiple
events, which is perhaps one of the reasons cancer is seen more often as we age. The first
mutation that is not repaired is thus inherited by any subsequent daughter cells. Cells thus
affected do not undergo apoptosis--cellular suicide--and are rendered immortal. (both
definitions are fine)
Even though immortalization is not the same as carcinogenisis, which is the generation of
cancer from normal cells, most transformed cell lines do not die after their normal
number of cell divisions. This is a requirement for the further development of
malignancies.
There appear to be several stages in the development of tumors. First, there is an
initiation phase, in which an optimum or threshold level of mutations occurs and “tips the
scales” toward tumor genesis. Once the cell has been transformed, there is a latent stage,
in which mutations that have a selection advantage start to proliferate. During the clinical
phase, the tumor becomes large enough to induce symptoms. These symptoms are caused
by tissue destruction, or the production of soluble factors that can be detected in the blood
or the tumor can depress vital functions and act as a space-occupying lesion in a confined
anatomical space.
GENETIC MISTAKES INITIATE CANCER
Since apoptosis is also under genetic control, it is not surprising that many of the protooncogenes and tumor-suppressor genes altered during apoptosis are those genes involved
with cell death. Many proto-oncogenes code for proteins involved in
mechanisms that regulate the social behavior of cells. Signals from those cells in the
immediate environment induce their neighbors to divide, differentiate and even undergo
apoptosis. It also appears that both types of genes are involved with or expressed during
40
the control points of the cell cycle. Human cancer studies show that mutations in the
tumor suppressor gene called p53 account for many tumors. One of the functions of this
gene is that it normally prevents cells with damaged DNA from proceeding through the
cell cycle. The presence of the protein product encoded by p53 induces the expression of
the waf-1 gene. The waf-1 gene produces a protein that normally inhibits the activity of
several similar cellular proteins called kinases (enzymes that catalyze the conversion of
proenzymes to active enzymes) that are involved in stopping cell cycle progression. A
mutation in either the p53 or waf-1 gene sometimes can cause the loss of that “emergency
brake” function and allow uncontrolled growth. One recent study has linked a case of
benign canine melanoma to loss of this function. However, loss of apoptosis isn’t the
only culprit that causes cancer.
Many types of genetic mishaps can occur and can lead to disease. The basic types of
genetic accidents include point mutations, deletions and chromosomal translocations
mentioned earlier. The insertion of mobile genetic elements such as transposons-segments of DNA that are capable of moving to a new position within the same or
another chromosome--or retroviral DNA-- retroviruses are potent disease agents with the
capability of incorporating their DNA into the host cell’s DNA--into the cell’s genetic
material are two other types of mutations.
Malignant transformations occur for a variety of reasons. Oncogenes, exposure to
chemical carcinogens and ionizing radiation such as X-rays all play a role in inducing
neoplasias. We even can “catch” cancer. In a number of species, although not yet
demonstrated in dogs, retroviruses have been proved to be the cause of a variety of
different diseases, including cancer.
A virus does not have the ability to reproduce itself but instead hijacks the host
cell’s reproductive capability by inserting its own DNA into the genome of the cell it has
infected. It then forces that cell to produce the proteins it needs. This can cause
something called an insertional mutation. Depending on where it inserts its viral DNA,
the mutation can wreak havoc in a variety of ways. The result of these genetic accidents
can alter the gene sequence so that it produces a protein with abnormal activity or even
no activity at all.
FREE RADICALS CAN ATTACK DNA
Outside influences also can lead to mutations and changes in cellular genetics. Because
we breathe an atmosphere that contains oxygen and we digest food, our bodies are
constantly producing free radicals--highly reactive oxygen molecules that occur
naturally in the body because of metabolic processes. Environmental factors such as air
pollution, radiation, pesticides, herbicides, many drugs and cigarette smoke react within
the body to cause free radical production. These molecules can damage DNA, affect the
structure and function of cell membranes and damage certain regions of proteins that
have enzymatic functions. Older humans and animals are more at risk due in part to
increased levels of free radicals as well as an impaired ability of the immune system to
eliminate altered cells. Very inbred dogs also have weakened immune function.
http://cc.ysu.edu/~helorime/inbrimmune.html
AUTOIMMUNITY & VACCINATIONS
41
Autoimmune disease is genetic but like many other polygenic diseases, there is an
environmental component. In the case of thyroiditis and diabetes, there is an established
link to environmental triggers. Why are we seeing a rise in such diseases in the
purebred dog? One could suggest it might be poor and outmoded breeding practices, i.e,
inbreeding referred to as line breeding by many dog breeders. The portion of the genome
that codes for the genes that help us recognize “self” is called the MHC--the Major
Histocompatability Complex. These genes are located very close to each other and
therefore it is very rare for recombination to occur. This in effect means that the genes
from each parent are inherited intact as haplotypes. If the parents are closely related, then
the possibility exists that they share the same genes at that site, i.e., they are homozygous
by decent. This essentially cuts the functionality of the immune response in half- not a
good thing. Normally, autoimmune diseases can be separated into diseases that are
“organ-specific” and “systemic” categories. For instance, an organ specific example
would be Graves’ disease that is characterized by the production of antibodies to the
thyroid-stimulating hormone (TSH) receptor in the thyroid gland. In the case of
Hashimoto’s, thyroiditis antibodies are formed against thyroid peroxidase; and in type I
diabetes (the type most often seen in dogs) by anti-insulin antibodies. An example of a
systemic autoimmune disease would be SLE (systemic lupus erythematosus). It also
appears that some individuals are more at risk than others of developing particular
diseases. As mentioned before, susceptibility to autoimmune disease is controlled by
environmental and genetic factors, especially MHC genes. Results from both twin and
family studies show an important role for both inherited and environmental factors in the
induction of autoimmune disease. In addition to this evidence from humans, certain
inbred mouse strains have an almost uniform susceptibility to particular spontaneous or
experimentally induced autoimmune diseases, whereas other strains do not. These
findings have led to an extensive search for genes that determine susceptibility to
autoimmune disease.
One way of determining this in humans is to study the families of affected patients; it has
been shown that two siblings affected with the same autoimmune disease are far more
likely than expected to share the same MHC haplotypes. The more closely related two
individuals are, the more likely that they share the same haplotype. The association of
MHC genotype with autoimmune disease is not surprising, because autoimmune
responses involve T cells, and the ability of T cells to respond to a particular antigen
depends on MHC genotype. It appears that susceptibility to an autoimmune disease is
determined by differences in the ability of variations of the MHC haplotypes to present
various proteins that mimic ”self” to those T cells that react to them. Inbred animals have
fewer allelic variants. An alternative hypothesis for the association between MHC
genotype and susceptibility to autoimmune diseases emphasizes the role of MHC alleles
in controlling the variety of T-cell receptors. This lack of diversity means that developing
and immature immune cells that are specific for particular self-antigens are not selected
against and so are allowed to reproduce themselves.
However, MHC genotype alone does not determine genetic susceptibility to disease.
Identical twins, sharing all of their genes, are far more likely to develop the same
autoimmune disease than MHC-identical siblings, demonstrating that genetic factors,
other than the MHC also affects whether an individual develops disease. One of these
genetic factors would be B or T cell immunodeficiencies. Symptoms of this condition
42
include eczema, dermatitis, heart disease, inhalant and food allergies and neurological
disease. These conditions are often seen in the purebred dog.
This however begs the question as to why we are seeing a rise in autoimmunity
associated with vaccinations. J Autoimmun. 2000 Feb;14(1):1-10.
Vaccination and autoimmunity-'vaccinosis': a dangerous liaison?Shoenfeld Y, Aron-Maor A.
The whole duration of immunity and the timing and necessity for various vaccinations
are
being questioned. Many veterinary training schools are changing their recommended
vaccination protocols. The practice of annual vaccinations lacks scientific validity or
verification. There is no immunological requirement for annual vaccinations. The
practice of annual vaccinations should be considered of questionable efficacy. Instead,
clinicians, in the absence of legal requirements, should educate their clients that an
annual physical examination is the better option.
ALTERNATIVES TO VACCINATIONS
Monitoring Serum Antibody Titers
As mentioned before, one of the “unknowns” with animal vaccinations is the duration of
immunity. What this means is that the pharmaceutical companies have not determined
how long a vaccination will protect against infection. One way to find out if an animal is
still protected is to measure the amount or level of antibodies to a particular antigen is
still present in the blood serum. This is called titering. Once an animal has been exposed
to a particular disease pathogen the body makes antibodies against that organism. After
they have done their “job”, some of those antibodies change and become dormant so that
the next time the animal is exposed to that same pathogen there is a stockpile of clones
that can jump in and fight off that same infection. It is these antibodies that are being
measured when an animal is titered. One point to consider is that titer levels do not really
reflect the ability of an animal to fight of an infection, but rather how recently they have
been “challenged” or exposed to that infectious agent. What this means is that the dog
that stays at home and is never around other dogs is more at risk than those that have an
active social life.
New Vaccination Protocols
Colorado State University
http://www.vth.colostate.edu/vth/savp2.html
UC Davis
http://www.vmth.ucdavis.edu/vmth/clientinfo/info/vaccinproto.html
University of Pennsylvania
http://www.vet.upenn.edu/comm/publications/bellwether/48/vaccination.html
University of Florida
http://www.vetmed.ufl.edu/sacs/Misc/2001vacprot.htm
Washington State University
http://www.vetmed.wsu.edu/rdvm/vaccine.html
43
Recent research may have uncovered the link between vaccinations and autoimmunity.
Most autoimmune disorders appear to be triggered by some type of toxic assault or a viral
or bacterial exposure. Why is this important? Preliminary studies have shown that
something called molecular or antigenic mimicry may be involved. This intriguing model
argues that the body is reacting to small protein-like fragments of the pathogen that are
homologous to normal cellular components. If true, this would be a form of antigen or
molecular mimicry in which antibodies formed against one molecule react with another
similar looking molecule. Another factor to consider is illustrated in a recent study that
showed that contaminants (specifically bovine thyroglobulin in rabies vaccine) that
remain from growth of the cells in culture during vaccine production cause the dog to
make antibodies against these contaminants. Since the bovine thyroglobulin molecule is
very similar to the dog's own thyroglobulin, the antibodies produced against the bovine
thyroglobulin cross-react with the dog’s thyroglobulin. This could explain why so many
dogs have thyroiditis characterized by high levels of antithryoglobulin antibody in their
serum. This may in turn, explain why we currently have an epidemic of hypothyroidism
in dogs in the United States.13
Vaccination Take Home Message
Vaccination should only be given at age appropriate times – the most common reason for
vaccine failure is maternal antibody interactions.
1. Never vaccinate a dog that is ill or malnourished (the second most common
reason for vaccine failure is nutritional deficits).
2. Only vaccinate for the: (a) “core” diseases like distemper and parvo, (b) those
diseases appropriate for your dog’s environment and (c) those mandated by law.
3. Follow the new guidelines for frequency of vaccinations and suggested
combinations of vaccines.
Chapter 7
AND WHAT OF THE FUTURE…?
The fancy has traditionally selected dogs for breeding programs based on arbitrary
conformation traits, rather than soundness of structure and overall health. In some cases,
form no longer follows function. Breeders and clubs tend to focus on a handful of traits-if that many--at the expense of the whole dog. As for genetics, the relatively simple days
of Mendel are not even a memory. Simple dominant or simple recessive genes do not
cause most of the disease problems we face in purebred dogs--they are usually polygenic
44
with most of the genes still unknown. What we should do revolves around the question of
how to pick a potential breeder. DNA testing is expensive and, at this point in time,
available for only certain diseases in certain breeds. The canine genome is not yet
complete, and even when it is, it will be many years before the combination of genes
causing diseases of interest are identified, and then years before tests are widely available
for those diseases. These are noble and necessary goals for sure, but how do they factor
into your life as a breeder? In truth, they don’t. A word of caution is warranted here: do
not hold your breath waiting for the magic test that will tell you whether or not to breed
your dog. Let’s look at practical and achievable solutions. There are two, and they have
been right under our noses all the time:maintain genetic diversity and share information
with each other. The goal of a breeder is to produce a “better” dog. What constitutes
improvement is at issue here. If you want to improve a breed, you must know the first
principle of evolution. Evolution, by definition, is change and diversification over time in
a species. However, if there is no genetic variability, there can be no evolution. Genetic
variability is the result of naturally occurring mutations and recombination.
MAINTAINING DIVERSITY
Maintaining genetic diversity will help control the expression of genetic disease, not
eradicate it. We are talking quality of life for the individual dog. Genetic diseases do not
skip from dog to dog as viruses do. You do not have to inoculate against genetic disease.
All you have to do to keep genetically transmitted disease out of your line is not to breed
affected dogs. Simply said but not easily achieved. Within this series of essays, we have
explored the mechanisms underlying the disease processes, and yet, even with new and
greater understanding, we have yet to find the solution to the problem. What it would
take is a series of relatively minor but wide reaching changes in philosophy, policy and
practice throughout dogdom--in essence, a paradigm shift.
First, clubs must talk to and educate their members, then achieve a cohesive consensus on
breeding strategies that reduce the number of affected dogs. Second, clubs must talk to
each other and form a coalition to fund studies that will lead to testing for genetic
diseases. Third, the AKC must be convinced or otherwise encouraged to participate in a
widespread reform and redefinition of breeding stock. Purebred registries should not be
scrapped—there is too much history and tradition supported by a huge pyramid base of
love and devotion to the dogs. What about adding new category: Breeding stock? Each
parent club will have to define what is meant by “breeding stock” for their breed. The
definition will involve a listing of the major genetic “faults” known in the breed. One
could classify genetic faults by their severity:
Class I - Severe traits would include painful or disfiguring disorders that maim or
otherwise cause the animal to be non-functional. Naturally, lethal traits or faults
that require medical intervention or treatment for the duration of the animals life
would be included in this category. Some few examples of this class: glaucoma,
craniomandibular osteopathy, hip dysplasia, entropion, portal systemic shunts,
cataracts, retinal dysplasia and detachment, PRA, deafness, dwarfism, inherited
kidney disease, diabetes mellitus, hypothyroidism, epilepsy, copper toxicosis,
ventricular septal defects, elbow dysplasia, and distichiasis.
45
Class II – This class would include genetic faults that are easily treated and
respond well to therapy, those faults that are corrected by one-time surgery that
could be considered primarily cosmetic, and those disorders that make the dog
unsuitable for the purpose for which it was bred. Examples of this class would
include bite misalignments, hernias, unilateral cryptorchidism, faulty dentition
and gait abnormalities. Dogs accepted into the Breeding Stock Register of each
club would have to be veterinarian-certified free of Class I or II faults.14
Examination and testing, consistent with current technology would be required as
necessary to screen for the diseases recognized in the breed. A number of safeguards
would have to be employed to ensure the purity of the Breeding Register. Absolute
identification of the animal to include DNA identification, supported by a tattoo and/or
microchip would be required. We are dealing with the breed’s future, so no bogus
registrations can be allowed. [Devil’s Advocate time: What about late-onset problems for
which there is no screening test? What about problems, like epilepsy, for which there is
no positive testing process even for the afflicted?]
It is easier to get information from dissatisfied buyers about what they bought than it is to
get information from breeders protecting their standing in the fancy. If breed clubs and
AKC were to cooperate, any person registering a puppy, would receive along with the
registration from the AKC, a postage-paid card to an open registry. If the puppy develops
a genetically transmitted disease, this card would be completed by a veterinarian and
forwarded to the registry.
Breeders might try to hide this information, much as they do radiographs of dysplastic
dogs they do not send to OFA and PennHip. On the other hand, puppy buyers, who feel
they spent good money for a product lacking in quality, would be more likely to comply.
It would take only one of the puppy buyers from a litter with an affected puppy to file a
report for the process to work.
The registries, if there were more than one cooperating with the AKC, would make their
information easily accessible. They could be independent of the AKC or under contract
or even be a division of the AKC. The important point is that the probability would be
high that should a genetic problem show up in a litter, that litter and the pedigree
supporting it would be flagged. Most puppies sold by breeders do not go to show homes
and other breeders where this information could be carefully hidden. While the process
might not be one hundred percent in 1 generation, over 5 or 10 generations, with a little
computer-aided backtracking and cross-referencing, almost every litter with carriers or
affected puppies could be identified. The nice thing about genetic disease is that if you
know which litters had an affected puppy, you know which dogs to test. It is much easier
and more economical to seek out, test and eliminate carriers if you know one of their
littermates was affected.
The more conscientious and ethical breeders would also inform the puppy buyers
and encourage them to report back any genetic faults encountered during the life of the
puppy or in any of its get. As breeders, we leave a standing challenge to parent breed
clubs, the AKC and the various registries to do something meaningful about the genetic
problems plaguing the purebred fancy. We are not talking about lip service—we are
46
talking about enlightenment, cooperation and action. Over the several decades that hip xrays have been done, the incidence rate of hip dysplasia in the general dog population has
been virtually unaffected. OFA and PennHip, both closed registries, have had minimal
impact; and CERF probably less because most dog people do not have their dog’s eyes
examined every year. What we need are open registries with on-line searchable databases
cross-referencing diseases to pedigrees.
ONE BREEDER AT A TIME
It should be noted that people, even including scientists, are not always prone to rational
facts. This especially applies to dog breeders. New and better information generally gets
absorbed slowly. In the case of diversity, we need to struggle with long held beliefs that
are no longer viable. Compounding the problem, in the case of inbreeding, these
outmoded practices produce conformation ring success, but at what untold cost? Those of
us who preach diversity find it very discouraging because these “old dogs” do not want to
learn new tricks. If one hangs about the breed ring long enough, the prospect of educating
breeders becomes overwhelming. However, instead of lamenting each incident and
outcome of the old ingrained inbreeding paradigm we need to take one step at a time and
rejoice in any and all progress. There is hope. It may be a human generation--and seven
dog generation--away! But hope, nonetheless. You read this book, didn’t you?
Appendix 1
MAPPING OUT THE DOG’S GENETIC FUTURE
Author’s Note: This chapter is highly technical and will be of most interest to
students of genetics.
In 1990, the greatest intellectual task ever attempted by humans began. Even more of a
challenge than walking on the moon, the Human Genome Map Project staggers the
imagination in terms of concept and complexity.
The ultimate intent of the project, completed in 2001, was to ascertain the definitive
sequence of the more than 3 billion base-pairs comprising the human genome. A genome
is all the genetic material in the chromosomes of a particular organism; its size generally
is given as its total number of base pairs. This enormous effort has “spilled over” to other
species, and dogs will reap the benefits. Building a road map of the dog’s makeup
through the Canine Genome Project eventually will lead to genetic tests that in turn may
eradicate many genetic diseases. These results, which haven’t the social, ethical and legal
implications that muddy the waters of the human genome work, may be used to enhance
the quality of our dogs’ lives and help us back out of the genetic cul-de-sac in which we
now find ourselves.
Pet owners spend billions of dollars every year diagnosing and treating genetic diseases
afflicting their pets. We now have in our hands the elementary tools to prevent or
ameliorate our dogs’ physical suffering. Recently, in a truly international effort, the dog
community took another small step forward: the publishing, far earlier than ever
expected, of the first canine linkage map. Not only will this endeavor help to discover the
basis for many genetic diseases in dogs, but the effort will spillover into human disease
also.
47
THE FIRST OF MANY HURDLES
One of the biggest hurdles to overcome when mapping a genome, human or canine, is to
assign a gene or genetic marker to a particular chromosome. Unfortunately assigning a
genetic marker has been much more difficult because most of the canine chromosomes
are the same shape and many are quite similar in size. Remember, besides the coding
regions (i.e. genes), chromosomes include noncoding regions within the gene that act like
spacers between the coding sequences. In addition, between the genes are long stretches
of noncoding areas. It is in these sections that Mother Nature has given us a gift to help
map the canine genome.
Interspersed along the entire length of the genome are regions called microsatellites.
These areas of DNA consist of tandem repeats (identical or nearly so) of a short basic
repeating unit, such as TGTGTGTGTGTGTG...ATTATTATTATTATT... etc. They can
be mono-, di-, tri- or tetranucleotide blocks, and are referred to as short tandem repeat
polymorphic (STRP) markers. Considered in evolutionary terms, these regions tend to
show a higher percentage of variations, therefore even closely related individuals will
exhibit differences. These variations can be as simple as a change of one base-pair, called
a point mutation, or as different as the deletion or addition of base-pairs.
For example, these repeats usually appear in blocks that vary from 10 to 30 units long. A
puppy could inherit a (TG)10 from its dam and a (TG)14 from its sire. If the pup carries
enough of these parental type alleles, it is possible to ascertain parentage. However,
further variations in additional markers would be necessary to differentiate between
siblings
IDENTIFYING GENETIC MARKERS
It has been suggested that it will require several thousand microsatellites to saturate the
canine genome. This means that there will be a marker about every 3 megabases (a
megabase is 1 million base-pairs). This will ensure that once these markers have been
identified, at least one of them will be associated with, and inherited along with, a
specific gene. Once a marker has become linked to a particular gene that has been
characterized for a specific trait or disease, it then can be used as a diagnostic tool to
screen for a desired characteristic. It would also be useful in identifying a carrier (or an
affected individual) of a genetically transmitted disease. This would be extremely
valuable information, as many inherited diseases are of the late onset type, meaning the
disease does not become evident until the dog is well past the age where it might have
been used for breeding.
NATURE’S SCISSORS
Another handy tool for the geneticist was discovered some 30 years ago. Scientists were
able to isolate several proteins from various strains of bacteria, named restriction
enzymes because they cut DNA at specific sites. The normal function of these enzymes
was to protect the bacteria from attack by phage (viruses that infect bacteria) or other
foreign DNA. Each restriction enzyme recognizes a particular double-stranded DNA
sequence. This specificity has been extremely useful for mapping the genome. Hundreds
of restriction enzymes have been isolated. Depending upon the source, these enzymes
“see” restriction sites that vary from four to eight base-pair recognition sites.
48
Some rare-cutter enzymes cut DNA very infrequently, which results in a small number of
very big pieces. The use of simple sequence repeats in identifying canine polymorphic
markers has been a fairly recent innovation. Prior to this, a technique called restriction
fragment length polymorphism (RFLP) markers were used to construct gene maps. Using
restriction enzymes that recognize base-pair sequences, it is possible to cut DNA into
various lengths. These segments can be separated by gel electrophoresis. DNA carries an
overall negative electric molecular charge. Under the influence of an electric field, the
different fragments migrate toward a positive charge at a speed that corresponds to their
molecular weight. Since the shorter fragments travel faster than the longer pieces, it is
possible by using this technique to differentiate between segments that differ by as little
as one nucleotide.
RFLP thus provides the basis for a technique called DNA fingerprinting that can establish
a parent-progeny relationship. The chief disadvantage of this procedure is that it is
extremely labor intensive (read: expensive) and requires a great deal of genetic material.
Tandem repeat markers have an advantage over RFLP because they can be assayed by
polymerase chain reaction (PCR) and have a higher polymorphic information content
(PIC). For this reason they have become the basis of the DNA parentage verification tests
in use today.
PCR is a technique that increases a specific section of DNA about 1 million times.
Since it is an automated procedure, the reaction can be repeated as many times as needed
to obtain ample DNA for that area being investigated. The DNA is then separated using
gel electrophoresis, and because the variations in length correspond to those of the repeat
sequence, it is possible to recognize individual differences. The main drawback of this
procedure is that the primers used in PCR amplification for a dog are not always the same
for other mammals, so unique markers must be developed for every species.
The term PIC is a little more complex. If a marker is to be useful, it must be unique. As
the number of variations within each marker increases, it becomes more and more
individualized and therefore has a higher polymorphic information content. This is a little
like saying my house is on First Street, then adding that it is on the corner of First Street
and B Avenue. If next I say it is on the northwest corner, it is easier to locate. Then if I
add that it is a white house with green shutters, etc., you can see that each little bit of
information increases the ability to find my house. It is these characteristics that make
markers useful for parentage verification and for the purposes of positive identification of
the animal. Remember it is possible to see chromosomes only in certain phases of the cell
cycle. The best time to see them is during metaphase. Normally, chromosomes exist in a
dispersed state that cannot be seen with an ordinary light microscope. Just before the cell
divides, it tightly gathers up its chromosomes. While in this state of metaphase, it is
possible to take a picture of all the chromosomes in the cell. As you recall this picture is
referred to as the karyotype, and in this picture, we can see the number of chromosomes,
their size and their physical appearance.
Standardization of the canine karyotype was necessary before researchers could
relate genes or genetic markers to their chromosomal origins. Development of
chromosome-specific markers will ensure that all of the canine chromosomes will be
represented within the map and that the linkage groups are correctly orientated. This
49
difficult barrier has been overcome by some brilliant work in England. Knowing the dog
has 39 haploid chromosomes (half of 78 is 39), researchers used a male dog in their
experiment because it is easy to see the Y chromosome. That left 38 chromosomes to
identify.
The researchers first separated the chromosomes by their DNA content and the use of two
fluorescent dyes that distinguish the base-pair ratio by preferentially staining either A-G
or C-T rich regions. Using a technique called dual-laser flow cytometry, they were able to
resolve their sample into 32 different components. Twenty-two of the portions contained
single chromosomes, and the remaining eight had two each. Thus, all of the
chromosomes were accounted for.
To identify the chromosome type, they then used these fractions to “paint” a normal
metaphase chromosome spread (a cell that has been “fixed” chemically so it no longer
cycles) and highlight the chromosomes using another technique called FISH (fluorescent
in-situ hybridization).
Hybridization is a very important concept to understand because it is the basis for many
of the methods used to study DNA. Recalling that the two possible DNA base-pairs are
C-G and A-T, if you have a DNA strand that reads AATGGCTAT, its complimentary
strand would have a base-pair sequence of TTACCGATA. In FISH, complementary
strands of DNA or RNA preferentially bind to each other. If one of the strands is tagged
with a fluorescent dye, it can be used to locate its equivalent complement on another
DNA strand. Other types of probes use radiographic or immunological labels. Hybrid
probes will be addressed in detail when we discuss mapping strategies.
MAPPING OUR WAY
Just like maps we use to find our way around town, genetic maps establish spatial
organization and symbolize a wide variety of information. They also are similar in that
there are different types of genetic maps, each with a corresponding range and level of
precision.
The karyotype (also known as a cytogenic map) is the lowest resolution of what is known
as the physical map. The highest resolution would be to know any posttranscriptional
modifications (changes in the RNA after DNA transcription) once we know the entire
base-pair sequence. Another type of genome map is a linkage map. The final genetic map
will be a synthesis of physical and linkage maps. This new map will let us know which
chromosome a gene is on, how many base-pairs separate each genetic marker, their
positions relative to each other and ultimately the complete basepair sequence. Once the
entire sequence has been resolved, we will need to find all the genes and use this
information to determine their function. The medical applications of this map alone are
overwhelming. These data, used as diagnostic tools to identify deleterious mutations,
combined with future gene therapy technologies, could lead to the eventual eradication of
genetic disease. We also could learn how certain behavioral traits are transmitted, which
is of especial interest to dog breeders.
MAKING THE LINKAGE
50
In 1865, the father of modern genetics, a young monk named Gregor Mendel, published a
paper in which he described the inheritance of certain traits he had observed while
growing peas. In choosing which attributes to follow, Mendel was very lucky that he
chose the characteristics he did, as they all turned out to be on different linkage groups.
As a rule, we equate linkage groups with individual chromosomes, and the number of
linkage groups corresponds to the haploid number of chromosomes. Thus, the dog has 39
linkage groups.
Mendel’s observations led him to postulate two “laws”. The first law says “particular
factors” (genes) come in different forms (alleles). When gametes are formed, these
alternative alleles are inherited independently from each other. Mendel’s second law
predicts that different genes (i.e., different traits that are not on the same chromosome)
will assort themselves into two different types of progeny in statistically equal amounts.
These two types are the parental type and the recombinant type. However, when the
genes that code for those traits are on the same chromosome, the percentage of
recombinant types would be less than the anticipated 50 percent. American geneticist
Thomas Hunt Morgan suggested this lowered recombination rate simply was a function
of how far apart the genes were from each other. The closer together they were, the more
likely they would stay ‘linked.’ We can use this information to predict the relative
distance between the loci of two genes. Today we measure the distance that separates
genetic markers in centimorgans (cM). Two loci are said to be 1 cM apart if they are
separated by a recombination event one percent of the time. This roughly corresponds to
a physical distance of one million base-pairs.
The next step in the mapping process is to determine the linear order of the genetic
markers. For instance, let’s say Gene A is 5 cM from Gene B and Gene B is 7 Cm from
Gene C. If we then find out Gene A is 12 cM from Gene C, we can assume their relative
positions are Gene A.....Gene B.......Gene C.
It would be nice if it were this easy. Unfortunately it is not. Coding regions, also know as
exons, are just too far apart to be linked conveniently, and so we need to use other types
of genetic markers. Another problem is that, compared with bacteria or fruit flies, the dog
has too few progeny to generate the statistical recombination data needed. Humans have
even fewer offspring.
The discovery and use of microsatellites, the genetic markers in the noncoding introns of
the gene, has overcome this barrier. So far, about 1800 canine microsatellites have been
characterized. At this time, markers have been found for 98% of the canine genome.15 In
order to be useful, these markers must be similar within species, breed and family groups,
yet be different enough (polymorphic) to detect the differences among individuals.
To repeat, microsatellites exist as di-, tri- and tetra repeat patterns, but because of
founder effect and the tight “linebreeding” inherent in the purebred dog, the most useful
microsatellites for elucidating the canine linkage map have been tetra repeats. Although
these areas are not genes, differences in the number of copies of the basic repeat unit also
are called alleles (length polymorphism). Because of technological advances, it is fairly
easy to ascertain the difference between two genotypes, and these procedures are the
51
basis for the most commonly used parentage tests now available. The more alleles a
microsatellite has, the more likely it is to be useful.
Mutations that occur within these regions do not cause changes in the dog’s appearance,
behavior or health; however, linking these genetic markers to disease alleles or genes that
characterize a specific trait will lead to diagnostic tests to identify carriers or affected
individuals. Several of these tests already are available. These microsatellites are
especially useful for identifying the carrier status of genetic disorders that arise from
mutations at different sites within the same gene. This is why breed-specific tests often
are required for the same disease.
Recombinant DNA technology promises to make higher resolution linkage maps
possible. A lab in France has used radiation to fragment human chromosomes and has
fused these fragments with cells from other species. These hybrid cells can be
manipulated so that only specific human chromosomal components are retained.
Determining the frequency of genetic markers that stay together after being irradiated
places their order and the distance between them at a finer resolution. These techniques
also have overlapped into the canine mapping effort. Work is progressing rapidly on a
radiation hybrid panel specific to the dog.
THE PHYSICAL MAPPING REALM
In addition to the linkage maps, there are several types of physical maps:
Chromosomal maps
Keep in mind that the lowest-resolution physical map is called a cytogenetic map
(karyotype). During the metaphase and the interphase stage of the cell cycle, it is possible
to stain the chromosomes with various dyes that result in distinctive banding patterns.
Using radioactive or fluorescent labels it is possible to assign genes or other identifiable
DNA fragments to their respective chromosomes and to estimate the distance between
them, measured in base-pairs. Improved FISH methodology now allows identification of
genetic markers from as close as 2Mb to 5 Mb apart (one Megabase, or Mb, equals
approximately 1 cM). With FISH, we can observe chromosomal mutations and
abnormalities associated with disease states. German researchers have discovered a
translocation on the first canine chromosome (a type of mutation—see Ch. 2) that is
linked to mammary tumors in dogs. Cytogenetic analysis may prove useful for
comprehending the underlying genetic mechanisms for other types of cancers for dogs
and humans.
Complementary DNA (cDNA) maps
Although two genetic markers may have a recombination rate higher than 50 percent, this
does not preclude them from being on the same chromosome. This further complicates
the mapping issue. The trick is finding out which of the 39 unique dog chromosomes to
assign a particular gene to. One of the methods used depends on knowing the protein the
gene is responsible for making, then working backward to figure out the approximate
DNA sequence. Using a tagged complementary hybrid probe made from a synthetic
DNA sequence, it is possible to see where the gene is located on the chromosome.
52
Another way to map a gene to a chromosome is to know the base-pair sequence of the
gene that codes for the same trait in a related species. For example, all mammals have
some genes in common. We even share conserved sequences—base sequences in a DNA
molecule that have remained essentially unchanged through evolution—with the lower
orders of animals. Although entire chromosomes are not conserved among species, parts
of chromosomes, called syntenic groups, are.
Homologous genes and genetic markers from the human mapping project have
been beneficial to the canine map effort. In turn, the canine map has been expected to be
useful to the Human Genome Project. Knowing the function or position of a certain gene
in one species makes it a possible candidate gene for the same ailment or trait in another
species. One such ailment, Severe Combined Immunodeficiency (SCID), is caused in
humans and canines by a mutation in one of the proteins that form the receptor site for
interleukin-2. Interleukin-2 is a chemical messenger that improves the bodies response to
disease. This specific defect causes a profound inability to mount both a cell-mediated
and humoral (antibody) immune response. It frequently is called the “Boy in the Bubble”
disease because of the movie about a boy with this affliction who lived in an isolation
bubble.
Several different laboratory techniques are used to localize differences to a smaller region
of the genome. Once such an area has been identified, it is possible to use automated
sequencing methods to distinguish any base-pair mutations. If these mutations result in an
amino acid substitution within the coded protein product, it becomes a likely suspect. The
candidate gene approach can save a lot of time, not only by providing a model for the
progression and course of a disease, but by suggesting treatment strategies.
Contig maps:
A better technique for obtaining finer mapping details is the contig map, produced by
cutting a chromosome into very small pieces, cloning these pieces and constructing an
overlapping clone “library.”
This kind of cloning is a recombinant technique that involves inserting a DNA segment
into another host cell, called a cloning vector, and using that cell’s own replication
apparatus to generate multiple copies of foreign DNA. This provides large amounts of
experimental material.
Cloning vectors often are bacteria such as E.coli, but recent technological advances have
made it possible to clone larger segments of DNA by using an artificial cloning vector
packed into a lamda phage. This virus normally infects bacteria and inserts its own DNA
into that cell’s genome, where it is replicated along with the normal cellular DNA. Using
nature’s own tricks has often worked well for us in this endeavor. Once contig mapping
of a particular section of the genome is accomplished in one laboratory, the resulting
genetic library can be published so other researchers can use the same information. A
common reference system called sequence-tagged sites makes this sharing of information
possible. STS are short DNA fragments (200 to 500 base-pairs long) whose unique base
sequence and location make them useful landmarks. A variation of this method is to
sequence cDNA partially instead of random sections of DNA. Complimentary DNA is a
special type DNA that is synthesized from a messenger RNA template. This is reverse of
53
the process as it normally happens. Since they are “tagging” a transcription product of an
expressed gene, they are called expressed sequence tags, or EST. These are especially
useful for finding candidate genes.
WE HAVE ONLY JUST BEGUN
Current mapping strategies, as brilliant and innovative as these techniques are, still have
left gaping holes that need to be filled. Genetic mapping is technology-driven, but
technology costs money and time. Faster, more precise mapping methods are needed.
Funding for the canine project generally takes a back seat to funding for the human
mapping effort and for species that are more agriculturally and economically important
than the dog. But it must be done as this information will provide us with the basis of all
genetic testing and strategies for coping with genetic disease.
Where is the money to come from? It is most likely that if genetic testing is embraced by
the dog fancy, market pressures will result in development of new tests, some of the
profits from which will be used to fund further research. If breeders do not test for
genetic disease, who will?
For breeds at great risk, certain breeding strategies such as introgression, a very complex
method that involves going back to the original stock and selecting for or against a
particular gene trait, should be considered. We need to contemplate opening up the
studbooks. At present, studbooks are closed. The only way to get back some of our lost
genetic diversity is to breed those dogs that made up the breed originally. The AKC could
have a set procedure for going back to the original foundation stock, even when there is
no actual breed registry in the original country. As it now stands, each breed club is being
asked to “reinvent the wheel” in this endeavor. The Samoyed, Saluki and the Basenji are
perfect examples of this predicament because seldom have tribal peoples from whence
these dogs came, maintained written records. Some hard questions also need to be asked
about the validity of our pedigrees and what must be done to protect those records. The
fancy must address these problems if our beloved animals are to have a viable future. We
are the custodians of our various breeds, thus the responsibility for finding answers to
these genetic problems is ours.
Appendix 2
GLOSSARY
Acrocentric – A chromosome with the centromere located close to one end.
Allele--Alternative forms of a genetic locus; a single allele for each locus is inherited
separately from each parent, each locus may have multiple alleles possible however only
two are available at a time, one from each parent.
Apoptosis – Natural process of cell death.
Assortative Mating – The mating of individuals that are phenotypically
similar. Assortative mating means mating like with like.
Autosomes--A chromosome not involved in sex determination.
54
Autosomal chromosomes—see Autosomes
Base pair (bp)-Two nitrogenous bases (adenine and thymine or guanine and cytosine)
held together by weak bonds. Two strands of DNA are held together in the shape of a
double helix by the bonds between base pairs.
Candidate gene – a gene which researchers feel is likely to be one which performs a
particular function because it performs a similar function in another species. Further
research will be required to determine if it is or is not the gene being sought.
Centriole – A structure that appears in pairs within the cell during the interphase portion
of cell division. During prophase the two asters migrate to opposite poles of the cell and
begin organizing spindle fibers which will guide the duplicated chromosomes toward the
asters prior to completion of cell division.
Centromere – A structure which joins two paired chromosomes together.
Chromatin--The tangled fibrous complex of DNA and protein within a eukaryotic
nucleus. See: chromosome.
Chromosomes-The self-replicating genetic structures of cells containing the cellular
DNA that bears in its nucleotide sequence the linear array of genes.
Co-dominant – Alleles of a gene which will both express in a heterozygous individual.
ExHumanB blood type.
Codon--The basic unit of the genetic code, comprising three-nucleotide sequences of
messenger ribonucleic acid (mRNA), each of which is translated into one amino acid in
protein synthesis.
Complete (simple) dominant – An allele which will be expressed in the phenotype even
if in a heterozygous pairing with another allele.
Constitutive heterochromatin – Condensed segments of the chromosome which are
transcriptionally inactive in all cells.
Contig map – A chromosome map that is formed by rendering chromosomes into small
pieces, cloning them then forming a “library” of overlapping cloned segments.
Cross Breeding – The mating of two recognized breeds to establish a new
variety or to improve an existing one.
Crossing over – See “recombination”.
Cytogeneticist – A scientist who studies cellular genetics.
Cytoplasm—The protoplasm of a cell exclusive of that of the nucleus, it consists of a
continuous aqueous solution (cytosol) and the organelles and inclusions suspended in it
and is the site of most of the chemical activities of the cell.
DNA (deoxyribonucleic acid)--The molecule that encodes genetic information. DNA is a
double-stranded molecule held together by weak bonds between base pairs of
nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G),
cytosine (C), and thymine (T). In nature, base pairs form only between A and T and
between G and C; thus the base sequence of each single strand can be deduced from that
of its partner.
Degeneracy – Term used to note that more than one codon can represent an amino acid.
Differentiation – Process of cellular development into different types of tissue.
Diploid--A full set of genetic material, consisting of paired chromsomes one
chromosome from each parental set. Most animal cells except the gametes have a diploid
set of chromosomes. See haploid
Disassortative Mating - The opposite of assortative mating, i.e., the
mating of dissimilar phenotypes.
Dominant--A gene is said to be dominant if it expresses its phenotype even in the
55
presence of a recessive gene.
Epistasis – A process by which the expression of one gene will prevent the expression or
influence the expression of another.
Exons--The protein-coding DNA sequences of a gene. See intron.
Euchromatin – Portion of the chromosome that is transcriptionally active.
Expression--The process by which a gene’s coded information is converted into the
structures present and operating in the cell. Expressed genes include those that are
transcribed into mRNA and then translated into protein and those that are transcribed into
RNA but not translated into a protein like mRNA.
Facultive heterochromatin – Bunched segments of the chromosome that are
transcriptinally inactive and which vary by cell type.
Founder effect – Changes in allele frequencies that occur when a sub-population if
formed from a larger one. Founders of the sub-population may have among them greater
or lesser percentages of particular alleles than is the case for the population as a whole.
Free radical--Highly reactive oxygen molecules that occur naturally in the body because
of metabolic processes which can damage DNA.
Gamete--Mature male or female reproductive cell (sperm or ovum) with a haploid set of
chromosomes.
Gene--The fundamental physical and functional unit of heredity. A gene is an ordered
sequence of nucleotides located in a particular position on a particular chromosome that
encodes a specific functional product (i.e., a protein or RNA molecule). See expression.
Gene frequency – Percentages of the different alleles of a particular gene that are found
in a population. Also referred to as “allele frequency.”
Genetic diversity – Percentage of genes that are polymorphic in a population; sometimes
refers to the proportion of genes that are heterozygous.
Genetic drift – Changes in gene frequency due to random chance (as opposed to
selection or mutation.)
Genome – The complete nuclear DNA sequence of a species, including all variations.
Genotype--The genetic constitution of an organism. See phenotype.
Haploid--A single set of chromosomes (half the full set of genetic material), present in
the egg and sperm cells of animals.
Haplotype – Set of linked genes on a single chromosome, usually inherited as a unit.
Heritability – Portion of phenotypic variation possible due to genetic inheritance as
opposed to environmental influence.
Heterochromatin – Bunched segments of a chromosome which are transcriptinally
inactive.
Heterozygous--Containing two different alleles of the same gene. See homozygous.
Homologous pairs--A pair of chromosomes containing the same linear gene sequences,
each derived from one parent.
Homozygous--Containing two copies of the same allele. See heterozygous
Hybridization – The crossing of two distinct strains, sometimes across species lines (i.e.
a mule.)
Imprinting--The expression of a gene determined by which parent it has been inherited
from.
Inbreeding – The mating of related individuals; also includes what dog breeders term
“linebreeding.” The technical definition is the breeding of animals which results in
progeny having a greater coefficient of inbreeding than is the average for the breeding
population.
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Incomplete dominance – Alleles that, when heterozygous, result in a phenotype
intermediate between those of the homozygotes of those alleles.
Interlocus – Reaction between genes, as in epistasis.
Intralocus – Reaction between alleles of a particular gene.
Introgression – The expansion of the gene pool of a population via the introduction of
unrelated individuals from another population (i.e. through imports or cross-breeding.)
Introns--The DNA base sequences interrupting the protein- coding sequences of a gene;
these sequences are transcribed into RNA but are cut out of the message before it is
translated into protein. See exons.
Kinase – A type of enzyme that that catalyzes the conversion of proenzymes to active
enzymes.
Karyotype – An individual’s chromosome complement; also the arrangement of
chromosomes at metaphase into a sequence ordered by length and location of the
centromere.
Lamda phage – A bacterial virus used as a cloning vector.
Line breeding - The mating of later generations back to some ancestor or
its descendents. Line breeding is a form of inbreeding.
Linkage – A measure of the frequency at which two genes on the same chromosome will
pass together to gametes.
Linkage disequilibrium – The tendency of alleles of closely linked genes to be inherited
together.
Locus (pl. loci)--The position on a chromosome of a gene or other chromosome marker;
also, the DNA at that position. The use of locus is sometimes restricted to mean regions
of DNA that are expressed.
Major Histocompatability Complex (MHC) – A group of genes that govern immune
system function, all closely linked on a single chromosome.
Meiosis – The process by which germ-line cells produce gametes.
Messenger RNA (mRNA)--RNA that serves as a template for protein synthesis.
Metacentric – Chromosomes in which the centromeres are located mid-way down their
length.
Methylation – The addition of the functional group –CH3. This plays a role in gene
expression and in post-transcriptional modification.
Microsatellite – Highly repetitive segments of non-coding DNA, often used for
parentage verification or as markers in indirect gene tests.
Mitosis – The process by which cells divide.
Monosomy – Occurs when an individual has inherited only one copy of a chromosome;
in mammals this is lethal not long after fertilization.
Mosaicism – The phenotypic effect of the random deactivation of one copy of the X
chromosome.
Mutation--A permanent transmissible change in the genetic material, usually in a single
gene. Also, an individual exhibiting such a change.
Natural selection - Process that results in adaptation of an organism to its environment
by means of selectively reproducing changes in its genotype. Variations that increase an
organism's chances of survival and procreation are preserved and multiplied from
generation to generation at the expense of less advantageous variations. As proposed by
Charles Darwin, natural selection is the mechanism by which evolution occurs. It may
arise from differences in survival, fertility, rate of development, mating success, or any
other aspect of the life cycle. Mutation, gene flow, and genetic drift, all of which are
random processes, also alter gene abundance. Natural selection moderates the effects of
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these processes because it multiplies the incidence of beneficial mutations over
generations and eliminates harmful ones, since the organisms that carry them leave few
or no descendants.
Neotany – The tendency of a species to retain infantile or juvenile characteristics.
Nondisjunction – An error of cell division that allows two copies of a chromosome to
wind up in a single daughter cell or gamete, ultimately causing trisomy or monosomy.
Nucleic Acid--Linear polymers of nucleotides. Nucleotides form the basic building
blocks of nucleic acids. They are made up of a nitrogen-containing purine or pyrimidine
base linked to a sugar (ribose or deoxyribose) and a phosphate group.
Nucleotide--A subunit of DNA or RNA consisting of a nitrogenous base (adenine,
guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a
phosphate molecule, and a sugar molecule (deoxyribose in DNA and ribose in RNA).
Thousands of nucleotides are linked to form a DNA or RNA molecule.
Nucleus--The major organelle of eukaryotic cells, where the chromosomes are separated
from the rest of the cell by the nuclear envelope
Outbreeding - A term generally taken to be the opposite of inbreeding.
It can be applied to outcrossing or cross breeding.
Outcrossing – Breeding from totally unrelated animals of which one is or
both are inbred (or linebred) within a given breed.
p – The short arm of a chromosome.
Penetrance – The frequency with which a genotype will produce a phenotype.
Phage – Viruses which infect bacteria.
Phenocopy – Different genes which produce similar phenotypes.
Phenotype--The physical appearance/observable characteristics of an organism. See
genotype.
Pleiotropic – The ability of a single mutation affect several traits.
Polygenetic – A trait resulting from the action of multiple genes.
Polymorphic – A gene which has multiple alleles. It also applies to non-coding regions.
Polypeptide--A peptide containing more than two amino acids and that are named
according to the number of amino acids they contain.
Posttranscriptional modifications--Changes in the RNA after DNA transcription.
Primary transcript--RNA transcript immediately after transcription in the nucleus,
before RNA splicing to form the mature mRNA.
Promoter – Regulatory section of DNA to which RNA polymerase binds prior to
transcription.
Protein--A large molecule composed of one or more chains of amino acids in a specific
order; the order is determined by the base sequence of nucleotides in the gene coding for
the protein. Proteins are required for the structure, function, and regulation of the bodys
cells, tissues, and organs, and each protein has unique functions. Examples are hormones,
enzymes, and antibodies.
q – the long arm of a chromosome.
Reading frame--A contiguous, non-overlapping set of triplet codoms in RNA or DNA
that begin from a specific nucleotide.
Recombination (homologous)--The exchange of DNA fragments between two DNA
molecules or chromatids of paired chromosomes (during crossing over) at the site of
identical nucleotide sequences.
Recessive--A gene that is expressed only when it is present in two copies or if the other
copy is missing. See dominant.
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Recombination – Process by which like segments of homologous chromosomes will
exchange places during meiosis.
Regulatory gene – One that is involved in the control of other genes.
Retroviruses--Any virus in the family Retroviridae that has RNA as its nucleic acid and
uses the enzyme reverse transcriptase to copy its genome into the DNA of the host cells
chromosomes. Many cancers in vertebrates are caused by retroviruses.
RNA (ribonucleic acid)--A chemical found in the nucleus and cytoplasm of cells; it
plays an important role in protein synthesis and other chemical activities of the cell. The
structure of RNA is similar to that of DNA. There are several classes of RNA molecules,
including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each
serving a different purpose.
RNA polymerase – RNA enzyme involved in the transcription of DNA.
Sex linkage – Inheritance patterns resulting from genes located on the sex chromosomes.
Simple dominance – See “complete dominance”.
Structural gene – One that encodes amino acids that ultimately lead to formation of
tissues or the regulation of body functions.
Submetacentric – Chromosome in which one arm is slightly longer than the other.
Suboptimal – An allele that causes slightly reduced function.
Syntenic groups – Genes on the same chromosome.
Template – Section of the DNA that is copied by RNA.
Transcribed (transcription)--The synthesis of a RNA copy from a sequence of DNA (a
gene); the first step in gene expression. See translation.
Translation-- The process in which the genetic code carried by messenger RNA directs
the synthesis of proteins from amino acids. See transcription.
Triploidy – A condition in which a cell or individual has three copies of a chromosome.
Trisomy--Occurs when an individual has inherited three copies of a chromosome; in
mammals this is lethal before or shortly after birth.
Uniparental disomy – condition in which an individual inherits two copies of a
chromosome from one parent and none from the other.
The Y chromosome - the small chromosome that is male-determining in most mammal
species. The male has one Y chromosome and one X chromosome. The Male Specific
region (MSY) comprises 95% of the chromosomes length and is made up of
heterochromatic areas (condensed during the interphase portion of the cell cycle) and
three 3 specific euchromatic (diffuse during the interphase portion of the cell cycle) areas.
So far only the euchromic areas have been found to contain transcription factors,
including coding genes. One region of the euchromic region is homologous to and pairs
with the X chromosome and is now called the x-transposed region. It contains 2 coding
genes that are expressed in the testis. The x-degenerate region is composed of
ancient remnants of autosomes (non-sex determining chromosomes) from which both the
x & y chromosomes evolved. It contains 16 coding genes that are expressed widely in all
the tissues of the body. (These genes have x-linked homologues) The amplionic region
contains 60 coding genes that are linked to male sexual development with regard
to sperm development and triggering appropriate hormonal output.
Appendix 3
"The Additive Relationship is the most commonly used measure of
relationship. It is a measure of the fraction of genes shared by two
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animals and thus is an indication of how reliable one of the relative's
records will be in predicting the genetic value of the other animal. The
Inbreeding Coefficient of an animal is calculated as one-half the
Additive Relationship between the parents." (p199, "Genetics for the
Animal Sciences," LD Van Vleck, EJ Polak, EAB Altenacu, 1987)
Additive Relationship is twice the Coancestry (which is also called the
coefficient of kinship or of consanguinity), "The coancestry of any two
individuals is identical with the inbreeding coefficient of their
progeny if they were mated. Thus the coancestry of two individuals is
the probability that two gametes taken at random, one from each, carry
alleles that are identical by descent." (p85, "Intro. to Quantitative
Genetics," Falconer & Mackay, 4th Ed., 1996)
The important difference between the COI and the coancestry (one-half
the additive relationship) is that one, the COI, refers to the
individual animal. The coancestry refers to the genetic similarity
between two animals.
A bitch "Jacki" might have a high COI. A dog "Jessie" might have a high
COI. But the coancestry of Jacki and Jessie might still be low or even
zero. If this is the case, then both Jacki and Jessie are inbred (in
fact, the entire breed line might be inbred), but there is still
significant genetic diversity in the breed line if Jacki and Jessie have
a low coancestry.
Continuing - If Jacki and Jessie have high COIs, they are prone to all
the problems associated with inbreeding. If the average COI for the
breed line is high, then the whole breed line is inbred and is likely to
suffer the consequences as reflected in rates of hereditary defectives,
increased puppy mortality, and reduced longevity. Yet, a low average
coancestry for the breed line implies that there is a lot of latent
genetic diversity in that same breed line. If so, the breed line can be
salvaged by the appropriate choice of mates.
So when you discuss the genetic health of a single animal, consider its
COI. When you discuss the latent genetic health of a breed (or a breed
line), consider the average coancestry. To compare coancestry figures be
certain that they are computed to the same depths of pedigrees (same as
with COI comparisons).
1
Robert J. Russell, Ph.D. e-mail communication 1998.
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Vila, Carles et al “Multiple and Ancient Origins of the Domestic Dog,” Science, Vol. 276, No. 5319,
June 13, 1997, p. 1687 (3)
3 Vila, C; Maldonado, JE; Wayne RK; Phylogenic relationships, evolution and genetic diversity of the
domestic dog. Journal of Heredity, 1999 Jan-Feb;90 (1): pps. 71-7
4 Watson, James D., The Double Helix: A personal account of the discovery of the structure of DNA, New
York: Atheneum, 1968
5 Brewer, George MD, e-mail communication, 1998.
6 Kealy RD, Olsson SE, Monti KL, Lawler DF, Biery DN, Helms RW, Lust G, Smith GK, “Effects of
limited food consumption on the incidence of hip dysplasia in growing dogs.” J Am Vet Med Assoc. 1992
Sep 15;201(6):857-63.
7 Daniel L. Hartl and Andrew G. Clark’s “Principles of Population Genetics. 3 rd 1977, Sunderland,
MA:Sinauer Associates
8 D John Armstrong, PhD, quoted by S. Thorpe-Vargas, DC Coile, JC Cargill “Ethics III ?” DogWorld, ?
2001, pp
9 John Armstrong, Ph.D. e-mail communication, 1998.
10 Hartl, ibid.
11 MB Willis, Genetics of the Dog, Howell Book House, 1989, pp.293-5.
12 CA Sharp, “The Biggest Problem” Double Helix Network News, Vol. XIII No. 3, Summer 2000.
13 Glickman, L In Press
14 Adapted from George Padgett’s Control of Canine Genetic Diseases, Howell Book House, 1998,
pp.161-2.
15 Personal communication Debby Lynch, AKC’s Canine Health Foundation, June 26 th, 2002
2
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