Biology 30
Module 3
Reproduction and Genetics
Lesson 13
Chromosomes and Genes
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
181
Lesson 13
Biology 30
182
Lesson 13
Lesson 13 Chromosomes and Genes
Directions for completing the lesson:
Text References for Suggested Reading:
Read BSCS: An Ecological Approach
Pages 130, 149-150, 169-182, 186, 209

OR
Nelson Biology
Pages 516, 575-578, 599-600, 605-606, 622-623, 662-663

Study the instructional portion of the lesson.



Review the vocabulary list.
Read additional Information on Pedigrees and Sex-linkage.
Do the Practice Problems.
Do Assignment 13.
Biology 30
183
Lesson 13
Vocabulary
autosomes
chromosome mutations
chromosome theory
crossing-over
deletion
fragmentation
frame shift
gene linkage
gene maps
gene mutations
gene pool
insertion
inversion
karyotyping
locus
multiple alleles
mutagens
Biology 30
nondisjunction
pedigree
point mutation
polygenes
polymerase chain reaction
polyploidy
Rh negative
Rh positive
sex chromosomes
sex-influenced
sex-limited traits
sex-linked traits
synapsis
tetrad
transposons
universal donor
universal recipient
184
Lesson 13
Lesson 13 – Chromosomes and Genes
Introduction
Transmission of characteristics from parents to offspring and the manner in which it
is accomplished, was largely a matter of speculation prior to 1900. The Austrian
monk Mendel, through breeding experiments and observations of peas, was able to
draw up some important conclusions about heredity in the mid-1800's. However,
after publication, his findings remained unnoticed until shortly after 1900.
Rediscovery of his works in the early 1900's, coupled with recent first-time
observations of chromosomes and the actions of mitosis and meiosis, quickly led
geneticists to new discoveries. This lesson will look in a more detailed way at just
what genes and chromosomes are and how they function. It will also examine this
information as it relates to ourselves, including the consequences of changes to genes
and chromosomes in their numbers or structures and in their actions.
Biology 30
185
Lesson 13
After completing this lesson you should be able to:
•
state some of the more important contributions made in
genetics by some scientists after Mendel.
•
relate the nature of chromosomes and genes to the DNA of cells.
•
explain how genes function in their control of body structures
and activities.
•
explain gene linkage, crossing over and the significance of
crossovers.
•
distinguish between multiple alleles and multiple genes, and
their effects.
•
define gene pools and discuss gene frequencies in populations.
•
describe how sex determination occurs in higher organisms.
•
distinguish between sex-linked, sex-limited and sex-influenced
traits.
•
discuss mutations – the kinds, the causes and the possible
effects.
•
describe some aspects of heredity as they relate to humans.
Biology 30
186
Lesson 13
The Chromosome Theory
The development of microscopic techniques, which allowed chromosomes to be seen
and their actions to be studied, enabled some scientists to use Mendel's findings to
draw additional conclusions. Independently of each other, (Walter) Sutton and
(Theodor) Boveri (both in the early 1900’s) were able to relate Mendel's factors, or
genes, to chromosomes. Their Chromosome Theory expressed:




That a chromosome is made up of many genes.
The idea that genes were located on, and carried by, chromosomes. The theory
also stated that the two genes controlling a particular trait were located on
separate, homologous chromosomes.
That paired chromosomes separated in meiosis. (This action would explain
Mendel's Law of Segregation). It also said that each sex cell has half the number of
chromosomes than that of a body (somatic) cell
Genes located on different chromosome pairs separate independently of each
other, since the pairing and splitting of the chromosomes occur randomly during
meiosis. By stating this, the Chromosome Theory helps to explain Mendel’s Law of
Independent Assortment.
Linkage and Crossing Over
The ability to see chromosomes led to more and more new observations and
conclusions. Organisms do not have very high numbers of chromosomes. Plants
generally range from having 7 to 24 pairs of chromosomes. Animals may have wider
ranges, but the average of the higher ranges still does not put chromosomes into
exceptionally high numbers. It became clear that the numbers of traits or
characteristics of most organisms greatly outnumbered the chromosomes. This led
to the conclusion that individual chromosomes must contain many genes. Since
then, it has been found that genes are located on chromosomes like beads on a
string. Some chromosomes contain genes for different traits numbering into the
thousands.
Studies and new discoveries are still being made, but
present estimates of the total number of genes on the 46
human chromosomes are from 30 000 to 40 000 proteincoding genes. Some even suggest 75 000 protein-coding
genes. The variations exist because of the different methods
used for ‘gene-finding’.
Biology 30
187
Lesson 13
Each of the many genes or traits occupies a very specific place or
locus on a particular chromosome. This means that the
sequence of genes on a chromosome also follows a definite
pattern. As an imaginary example, a certain human chromosome
segment could have the following genes in order: tongue-rolling,
long eyelashes, curly hair and skin pigmentation.
Locus – a
specific position
of a gene on a
chromosome
A Homologous
Chromosome pair.
The arrangements of genes in specific locations and sequences on chromosomes is
known as gene linkage. When two homologous chromosomes pair up during
meiosis, the gene pairs controlling the same traits end up being opposite to each
other on the different chromosomes.
When Mendel formulated his law or principle
of Independent Assortment, he was fortunate
to have chosen traits which were located on
different chromosomes. Chromosome pairs
separate randomly at meiosis and this means
that genes on different chromosomes would
follow independent patterns of separation.
For instance, it can be seen that a TtRr pea
plant can produce four kinds of gamete (egg
or sperm) combinations, if the genes for stem
length and seed type are located on different
chromosomes. (see diagram)
Further studies of other traits by geneticists
turned up figures or ratios that did not follow
Mendelian rules. It was speculated and
confirmed that Independent Assortment
cannot apply to genes or traits located on the
same chromosomes. If T and R were on one
chromosome and t and r on the other
homologous chromosome, there would only
be two possible kinds of gametes (rather than
4, if the genes were on different chromosome
pairs).
Biology 30
188
Lesson 13
Continued experimentation and observations often showed ratios of offspring that
were not consistent with Mendelian rules or gene linkage. Microscopic examinations
of actions taking place during meiosis revealed some interesting developments.
It was found that during Prophase I chromosomes come together to form
homologous pairs. This pairing of homologous chromosomes is called synapsis.
Each pair is made up of four chromatids, which is referred to as a tetrad. (The
duplication of each chromosome to form two chromatids happens during interphase
just before meiosis).
- Homologous
chromosomes
contain the same
genes, but there
may be different
forms of the genes
– different alleles.
As the homologous pairs move closer together, the chromatids pair up gene for gene.
They wind and twist around each other. Often the intertwined chromatids break and
exchange segments between nonsister segments. This exchange results in new
combinations of alleles (or genes) on a chromosome. It has been estimated that there
are at least two to three crossovers per pair of homologous chromosomes. This
process of exchanging chromatid segments between homologous pairs is called
crossing-over.
Biology 30
189
Lesson 13
Cross-Overs that occur in prophase I of meiosis are a
source of Genetic Variation Cross-overs can occur between several segments of the nonsister chromatids in the
tetrad. The importance of this crossing-over is seen once the gametes are formed.
When the chromosomes segregate during gamete formation, a gamete won't
necessarily have a chromosome that can be regarded as coming entirely from the
mother or entirely from the father. The resulting offspring will have a combination
of alleles that are not found, as such, in either parent. Thus, cross-overs are
important as a source of genetic variation, common in sexual reproduction.
A characteristic of cross-overs that was discovered and has been used by geneticists
to locate the approximate location of genes on the chromosomes is the fact that
cross-overs are more frequent between genes that are farther apart on chromosomes
than those which are closer together
Geneticists have been able to establish gene sequences and locations, according to
frequencies of cross-overs. Various testing techniques, including controlled crosses
between organisms and special cell cultures, are used to identify which particular
chromosomes are carrying certain genes. This has lead to the development of
chromosome or gene maps.
A genetic map that shows the location of genes on a chromosome is called a gene map
With new biotechnology, new methods for mapping genes have been developed. One
method called polymerase chain reaction (PCR) clones millions of DNA fragments in
a few hours. PCR is used to hi-light genetic markers that are throughout the genome.
A genetic marker is a fragment of DNA that has a known location and its
inheritance can be traced. A marker can be a gene, for example, or a piece of DNA
with no known function. Previously it was stated that genes that are close together on
the chromosome tend to be inherited together. From this markers are used as
indirect methods of tracking inheritance patterns that have not been identified.
Biology 30
190
Lesson 13
The Human Genome Project was initiated in 1990 and lasted 13
years. It was coordinated by the U.S. Department of Energy and
the National Institute of Health. It has been an effort to map and
sequence the entire human genome. By the summer of 2000 the
first draft was completed. Estimates put the human genome at
three billion base pairs. That’s a lot of information!
A genome is all
of the genes in
the DNA of an
organism.
In the mapping of the human chromosome 7, it has been found that chromosome 7:


contains approximately 1800 genes
contains over 150 million base pairs, of which over 95% have been determined
To bring the DNA sequencing just a little closer home – Scientists at The
Vaccine and Infectious Disease Organization (VIDO) at the The University of
Saskatchewan in Saskatoon – are in the process of mapping the genome of
livestock. Their research capacity includes not only genomics but also
pathogenesis, bioinformatics and vaccine development, formulation and
delivery.
Along with chromosome or genetic mapping, there is a technique of chromosome
study that has been mentioned in other sections. Karyotyping is a photographic
preparation showing all the chromosomes (or genome) of a particular organism. To
prepare a karyotype some cells of an organism are removed and placed into a
nutrient culture. A chemical, colchicine, is added. This chemical allows cells to
begin mitosis, but it breaks up or prevents spindle fibre formation. Chromosomes will
thicken and line up in the equatorial area of a cell. At this point, cells are fixed and
stained on a slide. Photomicrographs, showing the chromosomes fairly clearly, are
prepared. Finally, chromosomes are paired up with their partners (homologues) and
arranged into an order to make up an individual's karyotype. Karyotypes are used
for various kinds of studies, including checking for abnormalities.
Biology 30
191
Lesson 13
Chromosomes, Genes and DNA
So far, in this lesson and in the previous one,
chromosomes have been regarded as
thread-like structures upon which there are
points or genes which somehow determine
particular traits. At this time, the exact nature
of chromosomes and genes will be related to
the DNA (or deoxyribonucleic acid) described in
other lessons. Each chromosome is actually a
long, spiralling strand of DNA, some
ribonucleic acids, proteins and other
substances. Of these, the DNA is really the
focal point in heredity. Each double strand
DNA molecule consists of alternating units of
phosphate group and sugars connected by
nitrogen bases.
DNA molecule segment –
normal shape
Biology 30
192
Lesson 13
The nitrogen bases or nucleotides (of which there could be five different types –
depending whether it is DNA or RNA) form sequences of three. These nitrogen base
triplets or codons are responsible for identifying some of the 20 or so amino acids
which can be used to form proteins. A sequence of a number of codons on a DNA
strand, which directs the formation of one complete protein, makes up one gene.
A codon
Importance of Proteins
Understanding how genes actually control or transmit certain traits has to be
centered first on proteins and their importance to bodies.



Many proteins form the structural nature of bodies such as parts of cell
membranes, cytoplasm and organelles. The manner in which these proteins or
cell parts are put together ultimately determines the roles of certain cells in those
bodies. This is the basis of cell differentiation in more complex, multicellular
organisms.
Another very important general function of proteins is to form enzymes. Enzymes
regulate body activities which include metabolic processes, rates of reactions and
membrane permeabilities. In this way, enzymes are really determining how body
parts and entire organisms react to stimuli.
Proteins also take part in forming antibodies, so that even body defenses can be
traced to a genetic base.
Biology 30
193
Lesson 13
It is all these effects of proteins that are expressed as traits or phenotypes of
individual organisms.
The actual formation of a specific protein begins at a DNA segment (or chromosome),
where a number of codons make up a gene.
The nitrogen bases making up DNA and RNA strands can only be grouped in
particular pairings. This means that a very exact "blueprint" is sent out of the
nucleus by the codons of a gene. This "blueprint" is in the form of messenger RNA.
The sequence of events which then follows sees individual amino acids being
assembled in the cytoplasm according to "instructions" from the gene. A large part of
the understanding of this sequence of events can be traced to the work of two
scientists by the names of Beadle and Tatum, in the 1940's. They established the
idea that one gene produces one protein (structural or enzymatic).
Questions to be Answered
Knowing how DNA or genes control protein formation and phenotypes still leaves
questions to be answered. Every body cell (somatic cell) of a multicellular organism
usually contains the full complement of chromosomes and genes (or genome)
common to that organism. Yet, many genes express themselves only in certain cells.
For instance, the actual effects of genes controlling eye pigments occur only in the
irises of the eyes; the effects of genes for the color of plant flowers express themselves
only in flower petals. Other situations see certain genes expressing themselves only
after certain intervals of time (during an aging process) or when certain
environmental conditions prevail. Such behaviors seem to suggest that genes could
Biology 30
194
Lesson 13
possibly interact with each other, with certain substances in cells and also with
environmental conditions at certain times.
Multiple Alleles
The examples presented so far as illustrations of Mendelian heredity suggested
individual traits as having a possible maximum of two expressions. However, Mendel
and geneticists after him found that some traits or phenotypes were controlled by
more than two genes. Some individual traits were found to be affected by 3, 4 or
even more genes.
Phenotypes or traits affected by more than two genes are
said to be controlled by multiple alleles.
Multiple alleles of a particular gene trait occupy the same position or locus on a
chromosome. In any one individual, only two alleles are carried at a time (one on
each chromosome of a homologous pair).
A common example of multiple alleles is that of coat color in rabbits. The normal
colored (grey or brown) gene also has "partner" alleles expressing himalayan (white
with black points on the legs, ears and nose), albino (white, with pink eyes) and
chinchilla (a light grey). Combinations of the different alleles result in different
expressions. Normal colored is dominant to the other alleles and himalayan is
dominant to albino and chinchilla. These last two show incomplete dominance with
respect to each other when they happen to pair up.
Multiple Alleles in Humans
Multiple alleles in humans are commonly illustrated by blood types. Different
proteins carried on the surfaces of red blood cells, in the blood serum and in other
blood components, can categorize human blood into a number of different genotypes
and phenotypes. Original testing of human blood involved the use of rabbits. The
entrance of any foreign protein materials (or antigens) into any body causes that body
to form defensive proteins or antibodies for protection. Future invasions by the same
antigens will result in faster defensive reactions by the now present antibodies.
Injecting human blood into rabbits causes them to form two types of antibodies. The
work of Dr. Karl Landsteiner in the early 1900's recognized four major blood groups
in humans. He identified these as types A, B, AB and 0. (The following is a recap of
information from Lesson 7 page 160.)
Biology 30
195
Lesson 13
Type A blood - had protein (antigen) A on the blood cell surfaces and in the serum.
Rabbit blood would develop antibody A or anti-A, which would cause a clotting of A
type blood. This clotting is the major danger resulting from the mixing of different
blood types during transfusions. Clotting would block the tiny arteries and stop
blood flow thus leading to the death of an individual.
Type B blood - had protein (antigen) B on the blood cell surfaces which resulted in
the production of Anti-B test serum.
Type AB - had both proteins (antigens) A and B and caused rabbit serum to form
antibodies against both of these proteins.
Type 0 blood - did not carry any of the particular proteins (antigens) on the blood
cell surfaces and did not cause any reaction.
The gene I for human blood has three alleles that are identified by IA, IB, i.
The
The
The
The
genotypes for Type A blood are – IAIA or IAi
genotypes for Type B blood are – IBIB or IBi
genotype for Type O blood is – ii
genotype for Type AB blood is - IAIB
Facts to know about blood alleles:



Types A and B alleles are both dominant to the 0 allele. (IAi and IBi)
Types A and B alleles show incomplete dominance with each other. (IAIB)
Since alleles are always in pairs, with one coming from each parent, humans can
show six possible genotypes and four phenotypes. These are shown in the
following table. The table also indicates the antibodies produced in the blood and
in the blood serum of each blood type. (These should not be confused with the
identifying test sera.) For instance, a person with Type A blood or A proteins,
would normally produce antibodies against B proteins, if they were actually
introduced.
Blood
Genotypes
Blood
Type
Phenotype
Blood
Antibodies
I AI A
I Ai
A
Anti-B
I BI B
I Bi
B
Anti-A
I AI B
AB
None
O
Anti-A
Anti-B
ii
From the table, it can be seen that:
Biology 30
196
Lesson 13

a person with Type AB blood type would not possess any antibodies but has
antigen A and B present. Such a person should be able to receive any type of
blood in a transfusion. This circumstance would make the individual known as a
"universal recipient".
A Person with Type 0 blood type has no reactive proteins (no antigens present), Type
0 can be used for transfusions into any other blood type. A Type 0 person was often
referred to as a "universal donor". Present medical practices follow more strict
procedures in trying to have exact matches of blood wherever transfusions are
necessary.
The Importance of Blood Typing
1. It is necessary to know the blood type of an individual before he/she can receive a
transfusion. Giving an individual the wrong blood would cause the blood cells to
clump blocking arteries and causing death. Persons with Type AB blood may only
donate blood to individuals with blood Type AB. Despite being a universal donor,
a person with Type O blood will only accept blood from individuals with blood
Type O.
2. Blood typing can be helpful in determining parentage. It is not definitive but it can
narrow the field.
Example 1: A child has blood type AB. His mother is Type B, what would be the
blood type of the father?



The genotype of the child is IAIB.
The genotype of the mother is either IBIB or IBi. It could be either in this case. All that is
important is that she has ‘donated’ the IB to the child. (Remember the offspring receive a
gene from each parent for a particular trait.)
A father with type O blood (genotype ii) would be immediately ruled out because the only
genes he would have to give would be i’s and there are no i alleles present in blood type
Biology 30
197
Lesson 13

AB. The father would have to be either I AIA or IAi.
The phenotype of the father is type A
Example 2: A child has type O blood. The mother is type B. Determine the genotype
of the mother. Could we safely say that the father was type A?






Remember that offspring receive one gene for a trait from each parent.
The genotype of type O blood is ii. If the mother was I BIB she would not be able to give
an i gene to the child.
The mother must have at least one i to give the child, so the mother’s genotype has to
be IBi.
Following the same thinking process the father also has to have one i to give the child.
No, we can not safely say that the father is type A blood. We need to look at the other
genotypes to see if there are others that could give an i, and so have a type O child. In
fact a father with a genotype of ii (type O) is a possibility as well as a father who has a
genotype of IBi (type B) and an IAi (type A)
The possible fathers for this child could be type A (I Ai), type B (IBi), and type O (ii).
Rh Factor
Another protein associated with human blood deserves mention for its importance in
child bearing. It is not part of a set of alleles. This particular protein is found in
some individuals' blood cells and blood sera and is absent in others. It is called an
Rh factor because it was originally discovered in Rhesus monkeys. Individuals
carrying the particular protein are labelled as Rh positive. Individuals without the
protein are Rh negative.
Problems Can Arise
Complications could develop when Rh negative mothers are pregnant with Rh
positive children. Sometimes, during a first pregnancy and often during birth, some
of a child's Rh protein could enter a mother's circulation. The mother’s system
recognizes the foreign protein so her body begins to produce anti-Rh antibodies.
With subsequent pregnancies, there is a danger that the anti-Rh antibodies will enter
a fetus' circulatory system and begin destroying its red blood cells. This could result
in damage to internal body tissues and organs, including the brain, and possible
death (and abortion).
One of the treatments for this problem is to inject a substance containing anti-Rh
antibodies into a mother a few days after she has given birth. This destroys any
possible Rh protein that may have entered the mother's system, without the mother's
own immune system being aroused. This prevents the mother's immune system from
becoming prepared for further Rh invasions.
Biology 30
198
Lesson 13
Multiple Genes or Polygenes
Studies of some plant and animal traits have shown some variations which were
originally thought to be due to multiple alleles. Colors of eyes were once thought to
fit into this classification, with shades ranging from light blues to greens to browns
and almost blacks. It is now commonly accepted that there are multiple gene pairs
or polygenes for some traits such as eye colors. The cumulative effects of anywhere
from two to five or more different pairs of genes could affect some phenotypes. The
different pairs could be close to each other on the same chromosome or far apart and
possibly even on different chromosomes. Each pair of genes usually shows a simple
Mendelian dominant-recessive relationship between its partners, but the resulting
phenotype is due to the total or cumulative effect of all the different pairs. For
example, let us assume that there are four different gene pairs affecting eye
pigmentation. Each gene of a particular pair is responsible for the expression of a
certain amount of melanin or dark pigment. A dark gene (more melanin) is usually
dominant to a light gene (less melanin).
In the preceding illustration, each of the pairings is dominated by a gene for more
pigment. The total effect of all the pairings is to produce dark (brown) eyes.
On the following chromosome, one gene pair has both genes for more melanin while
the remaining pairs all show effects for less melanin. A person of this type may have
blue or bluish-green eyes.
Other polygenic or multiple gene traits are believed to be associated with skin
pigmentation, and body weight.
Biology 30
199
Lesson 13
Mutations
In the process of duplicating thousands of genes and many chromosomes during
mitosis and meiosis, mistakes or "errors" commonly take place. These sudden
changes, resulting in the appearance of characteristics quite different from parents,
are mutations.
Causes of Mutations
The exact cause for any particular mutation is not often known, but geneticists and
scientists now know of a number of mutagens or mutation-inducing agents. Some of
these are:
1.
2.
3.
4.
5.
6.
Ultraviolet radiation
Radiations – such as ultraviolet light, x-rays.
from sunlight can
Chemicals – which could include such agents as
affect the DNA in
mustard gas (used in World War I), colchicine.
one’s skin cells. It
Smoke and smog.
causes the cells to
reproduce rapidly,
Physical agents – which could include sudden
thus, causing skin
temperature changes.
cancer.
Some food preservatives (chemicals).
Certain drugs (e.g. thalidomide – caused deformities in developing
babies).
Mutagens may "confuse" nucleotides when they are pairing during DNA replication or
when messenger RNA is being formed. Some of the results could include wrong
pairings, or insertions or deletions of nucleotides.
Other mutagenic factors or agents could be involved in changes, but there are still
some uncertainties about these. These could include the possible effects of some
viruses or diseases (German measles or rubella), age or health of parents or parent
cells, and the diet or particular nutrients taken in.
Types of Mutations
Mutations could take place either in body cells (somatic cells) or in gametes
(reproductive cells). Those that take place in body cells affect only the organism
itself, while gamete mutations can be passed on to the offspring.
Biology 30
200
Lesson 13
Mutations could be classed as either one of two general types. They are:
A. Gene mutations
B. Chromosome mutations.
A. Gene Mutations
The more common mutations is that of gene mutations. A codon consists of three
nitrogenous bases (a triplet like AUG). A sequence of codons makes up a gene. The
codons direct the formation of amino acids into the one protein that the gene is
responsible for. A change of even one nucleotide of one codon could cause a change
in an amino acid in a polypeptide chain or protein.
1. Point Mutations
A change to a single base pair is known as a point mutation. This occurs in DNA
and is then reflected in mRNA as you see below.
Ways point mutations occur:
a. A base change could have one base being replaced by one that would not normally
be involved in the pairing. In the following diagram, adenine in one codon is
replaced by guanine during replication.
In this particular substitution, it just so happens that the nucleotide
sequence of GAG stands for the same amino acid, which is glutamate.
This point mutation will therefore be insignificant.
b. Often, changes drastically alter the type of protein or enzyme formed. The protein
could change in form or in function, or both. The following codon change has
uracil being replicated in the place of adenine.
Check back to Lesson 10 on the codon wheel (or on the Genetic Code Table) for
which three letter codes code for which amino acid.
Biology 30
201
Lesson 13
Result: the new codon GUA codes for a new amino acid, Valine.
The new codon GUA changes the amino acid to valine. This changes the
entire protein.
Valine taking the place of glutamate in a protein is just how sickle-cell
anemia apparently developed. Colorblindness and hemophilia are two
other human disorders developing from point mutations where
substitutions occur.
c. Another type of point mutation could have a substitution changing a codon from
one which identifies a specific amino acid to one which stands for an end codon.
That is, the (end) codon signals a completion to the formation of a certain protein.
The formation of an end codon in a place where it does not normally
appear would mean that a complete protein will not be formed. This
could change the function of what had been formed so far or, more likely,
cause it not to function at all.
Biology 30
202
Lesson 13
2. Frameshift Mutations
In addition to base substitutions, mutations also occur when bases are either
inserted into, or deleted from, codons. The following illustrates a portion of a
polypeptide chain or protein with a number of its codons.
Insertions or deletions are sometimes called "frameshift" mutations. They have the
effect of changing the "reading frame" for messenger RNA. Remember that
nucleotides are "read" in series of threes. In the preceding illustration, the sequence
of UUC ACU CAC AGU stands for the amino acids indicated below each of the
codons. Insertion of the base adenine between uracil and cytosine of the very first
codon changes the reading frame.
a. Insertion of the base adenine shifts the entire reading frame or codon sequences
from the point of insertion. Reading the "new" codons results in completely
different amino acids and a different structural protein or enzyme is synthesized.
Such a new protein or enzyme will either not function or will function differently
from the previous one.
b. Deletion of a nucleotide base would also cause a frameshift mutation and a
similar result.
Biology 30
203
Lesson 13
The sometimes slight changes in phenotypes caused by gene mutations are believed
to be largely responsible for the development of multiple alleles. Multiple genes may
also have developed from gene mutations. Other gene mutations, such as sickle cell
anemia or hemophilia, can have much more serious and sometimes fatal results.
B. Chromosome Mutations
The second general type of hereditary change or "error" is that of chromosome
mutation. Chromosomal changes usually have more drastic effects than gene
mutations. The reason for this is that many genes tend to be involved. Most
chromosome changes happen during mitosis or meiosis, when chromosomes or
chromosome pairs are being separated.
1. There could be nondisjunctions, where some chromosome pairs fail to separate.
Gametes and new individuals could end up with higher or lower chromosome
numbers than is common for their species. In plants, polyploidy (where
individuals could have 3N or 4N numbers of chromosomes) happens quite
frequently. Many polyploid plants show more vigorous developments in roots,
stems and leaves.
Most animal embryos with chromosome numbers different than normal fail to
develop. Those that may develop and are born, unlike plants, frequently show
shortcomings in structures or behaviors.
In humans, nondisjunction of one particular pair of body chromosomes (pair number
21) results in an individual having 47 chromosomes where a section of a chromotid
breaks off and joins with its sister chromotid. Such a person is afflicted with Down's
syndrome, characterized by such traits as mental retardation and a shorter life
expectancy.
2. Chromosomal mutations can also occur with fragmentations. Some chromosome
or DNA strands split in one or more places. Fragmentations are often associated
with (chromosomal) deletions (where some of the chromosome is left out) and
insertions (where a section of a chromotid breaks off and joins with its sister
chromotid), where the chromosome pieces are removed from some parts of
chromosomes and possibly "spliced" into other parts of the same chromosomes.
Sometimes, the chromosome fragments are inverted when part of a chromosome
breaks off and is reinserted backwards.
Biology 30
204
Lesson 13
Examples:
a. Deletion
b. Insertion
c. Inversion
d. Transposition
Often, fragments are transposed or translocated from one chromosome to a
completely different one. That is, they go from one part of a genome to another.
People involved in chromosome studies sometimes call certain chromosome
fragments, which have a tendency to move more frequently, as "jumping genes" or
transposons.
Biology 30
205
Lesson 13
Significance of Mutations
A large majority of the gene or chromosome mutations which take place are harmful
or, at least, of no value to organisms. Since most mutant genes are also recessive
types, their negative effects could be "masked" or covered over by the "normal",
dominant genes. The negative effects come about when two parents carrying the
mutant recessives cross and an offspring inherits a homozygous recessive condition.
A change in phenotype may bring about what may be a relatively insignificant effect.
On the other hand, some could be quite noticeable or drastic.
For example:




The change in a single amino acid in a protein chain has led to sickle cell anemia
in humans. Red blood cells lose their normal round shape and the ability to carry
oxygen, which can be fatal.
Another gene mutation can lead to the absence of a particular functioning
enzyme, which results in albinism. Individuals lack normal pigments in their
skins and eyes (appearing white or blondish, with pink eyes). Albinos, especially
in natural or wild conditions, are faced with poor survival chances.
Other mutant genes in homozygous conditions can be either lethal or
semi-lethal, directly leading to organisms' deaths before they are born or before
they reach reproductive maturity.
Chromosomal mutations can result in more dramatic changes. In animals, many
of these result in unfavorable consequences, with major physical deformities, poor
brain (senses and intelligence) developments or early deaths.
A change in phenotype may bring about positive effects.

Even though most mutations are either harmful or of no benefit, some forms are
definitely beneficial either to individual organisms or to species. Polyploidy in
many angiosperm plant species often results in more vigorous developments such
as in growths, flower and fruit productions and resistance to adverse conditions.
This can be done by administering the chemical colchicine to plant cells.

Other chromosome and gene mutations in plants and animals could bring about
structural or enzyme changes to make organisms better suited to environmental
conditions at the time. Such changes or variations are particularly important for
the survival of species over time. Changing environmental conditions make it
important that species continue to experience variations. This enables forces or
agents of natural selection to "choose" individuals, which may be better adapted,
that will survive and will continue to reproduce. Then, as environmental
conditions do change over periods of time, individual species at least have
opportunities to continue to avoid extinctions.
Biology 30
206
Lesson 13
Gene Pools
Any organism in a species population of a certain area has the potential to show a
diverse number of traits or phenotypes. The possible genotypes and resulting
phenotypes are related to the possible parental combinations which can occur within
a population. The extent of the diversity among species members really depends
upon the numbers of different genes that exist in that population.
All the different kinds of genes existing in a population
collectively make up what is called a gene pool.
Genetic studies have shown a characteristic common to many gene pools. This
characteristic is called The Hardy-Weinberg principle.
The Hardy-Weinberg principle, states that in most natural
populations, the frequencies of particular genes tend to
remain the same.
Exceptions to the Hardy-Weinberg principle do occur. Some of these exceptions are:



A changing environment could bring about a process of natural selection in which
traits unsuitable to new conditions are gradually eliminated.
Mutations can also introduce new genes and change gene frequencies over periods
of time.
A major factor changing gene pools and gene frequencies within those pools
presently has to do with human activities.
Gene pools, as long as they remain localized and relatively isolated from each other,
are relatively unchanging. However, human activities are resulting in the transfer of
many plant and animal species from one area of our planet to another. Plant and
animal scientists, breeders, producers, as well as pet owners, flower enthusiasts and
others, are all responsible for such movements. When these happen and
interbreedings with local populations take place, gene pools change. Even humans
themselves are not exempt from this, as immigrations and intermarriages go on.
We will look more closely at the Hardy-Weinberg principle in
the next Lesson.
Biology 30
207
Lesson 13
Sex Determination
Among the more complex multicellular plants and animals, it was eventually
recognized that a chromosome pair was related to sex determination. Chromosomes
line up as homologous or matching pairs prior to meiosis. Early studies showed that
in female insects and mammals, nearly all chromosome pairings were matched.
However, male individuals showed something else. In the chromosome pairings,
there was a consistent appearance among males of one unmatched or
nonhomologous pair. One of the chromosomes was distinctly different in structure
(having a kind of "hook" at one end) from the other chromosome. These sex
chromosomes were identified as X and Y. A female, obviously having a matched
pair, has an XX combination. A male, with a dissimilar pairing, is XY. The other
chromosomes or chromosome pairings were given the name of autosomes or body
chromosomes. They carried genetic traits related to body developments other than
sex determination.
Once it had been determined that a pair of chromosomes was responsible for
establishing the sex of an organism, other generalizations were made. Using a
Punnett Square, one can see how two of these generalizations could have developed.
Suppose the events of sperm and egg formation are followed through to the
fertilization process.
When these combinations are plotted onto a Punnett Square to see the possible
results during fertilization, one would see the following:
Biology 30
208
Lesson 13
Generalizations:
1.
With or without the square, one may see that it is the male gamete that
determines the offspring's sex. The female can contribute only an X to her egg.
The male sperm can have either an X or a Y. Therefore, if the X sperm
fertilizes the egg, the offspring will be XX and a female. A Y sperm fertilizing an
egg produces the XY combination and a male offspring. Lest human females
feel slighted at seemingly being left out of the "decision" process, there is
evidence to suggest that some may have an effect. Certain (chemical)
conditions within a female reproductive tract could favor one or the other of the
two sperm types. However, it is still the sperm type that ultimately decides the
sex.
2.
Another generalization about sex determination concerns probabilities about
offspring being female or male. Again, with or without the Punnett Square, one
could see that the chances for a female or a male at any particular time are
50:50 or 1/2. In natural populations, the actual ratio of male to female
offspring tends to be a little higher on the male side. This seems to balance out
in that male embryos and young male offspring have higher mortality rates
than females.
Mammals and many insects have the arrangements whereby females have the
homologous sex pairings while males are XY. In birds, snakes and amphibians, this is
reversed, with females having the unlike pairs. In these groups, geneticists have
labelled the sex chromosomes pairs as ZW and ZZ, to reduce possible confusion with
mammals and insects.
In earlier discussions on how mutations can occur, one probable cause is described
as an abnormality in chromosome separation during meiosis. Such abnormalities
can also take place with the sex chromosomes. Studies have turned up individuals
having sex chromosome combinations other than those described. In all
combinations it was found that the presence of at least one Y was sufficient to
establish male characteristics. However, varying X and Y numbers and body
chromosomes (autosomes) in individuals could establish what are called supersexes
or intersexes. Individuals having only one X (and a total of 45 chromosomes) are
often shorter, somewhat mentally challenged, sterile females. A triple XXX female (47
chromosomes) is normal looking and is usually fertile. Males having two X
chromosomes, but at least one Y, are also normal looking but are generally sterile.
Gaining extra sets of autosomes or the non-sex chromosomes seems to bring about
significant interactions with the sex chromosomes, resulting in varying appearances
and fertilities.
Biology 30
209
Lesson 13
Sex-Linked and Sex Influenced Traits
It is believed that the sex chromosomes developed from a pair of body chromosomes
or autosomes. Genes for female characteristics were located on the X chromosome
(W in birds, reptiles and amphibians), while those for male traits came to be on the Y
(Z, in the other groups). The X chromosome appears to have remained much like
many of the other body chromosomes. It contains many genes for body
characteristics other than femaleness. On the other hand, the Y chromosome has
not only changed in appearance (becoming smaller and with a hook at one end), but
also seems to have lost many of the traits carried by the X.
Some geneticists occasionally call the Y an "empty" chromosome because it lacks
many of the genes carried on the X. However, it does carry some genes in addition to
the ones for maleness.
The full significance of the difference between X and Y chromosomes becomes
apparent in the study of sex-linked traits. Sex-linked traits are those phenotypes
caused by genes carried on one of the sex chromosomes – usually the X. In a female
(XX), a particular allele on one X chromosome could be "masked" by a dominant
allele on the other X chromosome. In males (XY), having no matching alleles on the Y
means that most alleles on the X will be expressed. (Note: most of our sex-linked
traits are on the X chromosome. Very few traits on the Y chromosome are known.)
To do a Punnett Square involving traits that are sex-linked, the traits are
shown on the X chromosome.
Example: A dominant trait is shown as a capital letter superscript – XC
The recessive trait is shown as the lower case of the same letter, as a
superscript – Xc.
The possible genotypes of an individual would be written as:
XCXC
XCXc
XcXc
XCY
XcY
is a normal female that does not carry the sex-linked trait
is a carrier female, the sex-linked trait is hidden
the individual carries and shows the trait
is a male that does not show or carry the trait
is a male that shows the trait
If you choose a letter for the dominant trait that looks the same in lower
case and when it is capitalized, like the letter c or s, you MUST make a
size difference when placing it on the X.
Biology 30
210
Lesson 13
Example of a sex-linked cross
A common example of sex-linkage and its significance is seen with respect to
hemophilia or bleeder's disease. A recessive gene causes a condition in which a
certain protein necessary for blood clotting is not produced. A hemophilic person will
bleed profusely from even small surface wounds and could actually bleed to death.
An X chromosome carrying a normal, dominant gene for blood clotting could be
depicted as XH. An X chromosome with the recessive, hemophilic trait would be
shown as Xh. A Punnett Square would show the implications of a heterozygous
female - normal male cross.
The cross is:
heterozygous female
XHXh
×
×
normal male
XHY
Note: The male
gametes are placed
in the spaces on
the top row of the
Punnett Square
and the female
gametes are placed
in the spaces down
the left side of the
Punnett Square.
Results:
All of the females will be normal, but 50% of them will have the possibility of being carriers.
50% of the males will be normal while the other 50% will have the trait. Read below for the explanation
for why they do.
Looking at the results, all the daughters will be normal, although half (like their mother) could be
carriers. Of any possible sons, one-half could be expected to be hemophiliacs. The gene for
hemophilia carried on the X chromosome, which comes from the mother, cannot be countered or offset
by any allele on the male's Y; therefore, the hemophilic phenotype shows up in a male.
See Practice Questions at the end of this lesson for practice
working through other sex-linked traits. e.g. colour blindness
Male offspring actually have to be more concerned with some of their mother's
genotype rather than the father's, since the X chromosome they inherit always comes
from her.
If one were to look at other possible combinations of parent crosses, additional
generalizations could be made:



Although hemophilic females are not common, they are possible if their father is a
hemophiliac and the mother is at least a carrier.
Sons have no chance of escaping a certain trait like hemophilia if the mother is
homozygous for the condition.
Mothers can transmit sex-linked traits to both daughters and sons, since both
sexes inherit one of her X-chromosomes.
Biology 30
211
Lesson 13

Fathers can only transmit sex-linked traits (on the X chromosome) to daughters –
since a son can only inherit the Y from a father. With respect to this last
characteristic, a male could transmit traits to his grandsons through a daughter.
The fruit fly, Drosophila melanogaster, is used by geneticists to study
Mendel’s principles of inheritance. One such geneticist, Thomas Hunt
Morgan, linked eye colour of the fruit fly to the x-chromosome. These were
sex-linked traits.
Fruit flies are very small and have a life cycle of 10 to 15 days. They have a
large number of offspring that make it fairly easy to determine ratios, etc.
The male fruit fly is smaller than the female so it is easy to tell the
difference.
Other common sex-linked traits in humans include red-green colorblindness,
muscular dystrophy and possibly a certain kind of baldness.
Among other animals, an interesting example has to do with coat color in domestic
cats. The X-chromosomes of female cats can carry genes for yellow (or yellowishorange) or black. Previously, it was considered that a heterozygous condition in
females (XYXB) expressed itself as an example of codominance, with both colors
appearing in patchy networks in a so-called tortoise-shell or calico effect. Males
seldom show this yellow-black spotting effect, since they inherit only one X
chromosome and can therefore show either yellow or black, but not both.
Occasionally, a male does appear with the trait. Such individuals are sterile and are
believed to be XXY. A more recent explanation is that often, in females, only one of
the X-chromosomes is active in a cell early in its development. The other X is
inactivated, possibly condensing into a ball-like structure (called a Barr body), in the
cell. Just which X chromosome is active in a cell could be a matter of chance.
Therefore, in some body areas or cells the traits carried by one X may be obvious
while in other body areas or cells it is the other X's traits. This can create mosaic
patterns over bodies for certain traits. See Practice Questions section at the end of this
lesson for an example to work through (#3).
In humans, a disorder caused by an allele that prevents sweat gland formation may
be expressed this way. A male who has this allele on his X chromosome has no
sweat glands. A heterozygous female has some body areas with sweat glands and
other body areas that do not.
Biology 30
212
Lesson 13
Pedigree Charts
Studies of sex linkage and family histories can often be simplified by using pedigrees
or charts. These show relationships among a number of generations within a family.
The following very simple pedigree traces colorblindness through a number of
generations. Note the key on the right hand side.
Question:
Explain why parent D is a carrier but parent C does not show the colorblind trait.
Give the genotypes of individuals A,B,D,E, and F.
Answers
Parent D (a female) receives her X from her father who is colorblind X c Y ). That is the only information
for that trait that he can give. Parent C receives his X from his mother who only has the dominant
gene on her X’s XC XC  .
The genotypes are:
A
B
C
-
D
E
F
Xc Y
XC XC
XC Y
-
XC Xc
Xc Y
XC Xc
Traits affecting many sexual characteristics are also carried on body chromosomes or
autosomes, in addition to being on sex chromosomes. These traits behave differently
than those that are sex-linked. The expression or appearances of some traits seem to
be determined by, and limited to, the sex of the person they are found in. These are
sex-limited traits. Sex-limited genes are strongly influenced by the amount and
types of hormones present in the body. Heavy beard, for instance, is a sex-limited
trait in humans. A male who has the allele pair Bb (heavy beard-light beard) will
have a heavy beard. However, in a female this particular genotype will not usually
express itself in any distinctly obvious phenotype. A female may show more facial
hair growth if she is homozygous BB.
Biology 30
213
Lesson 13
Certain other traits are regarded as sex-influenced. These particular body features
appear in one sex more than the other; however, they are not exclusive to a particular
sex. Baldness is much more common in males. A male having the genotype BB or
Bb will develop baldness. Some females can, and do, develop baldness. They will
experience this condition if they are BB. Even then, the condition in women is
generally not as noticeable and could involve just a gradual hair thinning with age.
Some Genetic Conditions-Disorders in Humans
As a conclusion to this lesson, an examination will be made of some human genetic
defects, including more on some that were already mentioned.
Sickle Cell Anemia
Sickle cell anemia is found in people of African descent and in people whose families
originated around the Mediterranean Sea. Sickle cell anemia arises from a gene point
mutation. The "instruction" in one nucleotide base of one codon, which changes the
identity of one amino acid in a sequence of over 500 others, causes this problem.
The amino acid alteration causes a change in the hemoglobin protein on red blood
cells. Instead of being disk-shaped, red blood cells are sickle-shaped. These
sickle-shaped cells cannot carry oxygen very well. They also have tendencies to block
blood vessels and to break apart easily. Individuals with this condition suffer anemia
or shortages of oxygen to body areas, as well as frequent internal bleeding. The
sickle-cell trait appears to be codominant with the normal hemoglobin gene.
People homozygous (HbSHbS) for the condition usually die before reaching adulthood.
About one in twelve African American is heterozygous (HbAHbS) for this condition.
People, who are heterozygous, have a mixture of normal and sickle shaped blood
cells. These individuals do suffer some of the pains and conditions of anemia, but
not to the extent that homozygotes do. People who are heterozygous for the sickle
shaped cells have a resistance to malaria (common in tropical countries) that people
who are not do not have. The sickle cells appear to be able to destroy the parasite
capable of causing malaria.
Huntington's Disease
A dominant body or autosomal gene is responsible for Huntington's Disease. This
particular gene results in the production of a substance that interferes with certain
brain functions and muscle co-ordination. A person eventually loses muscle
functioning and also suffers mental deterioration, before experiencing an earlier than
usual death. The symptoms and effects of Huntington's do not generally appear in
an individual until 40 years of age or more. Therefore, an unfortunate aspect to the
disease is that the affected individual may have had children by then – half of whom
could have the disease.
Biology 30
214
Lesson 13
Nondisjunction
In previous descriptions of meiosis and sex determination, mention was made of the
possibilities where abnormal chromosome separations could occur. Chromosome
pairs may fail to separate in the formation of eggs or sperm. Gametes that result from
this could either have higher or fewer chromosome numbers than normal and
individuals could also be born with these abnormal numbers. Frequently, disorders
arising from such abnormalities are called syndromes.
Down Syndrome
Down syndrome appears more frequently in children of older mothers. During egg
formation, a pair of particular body chromosomes fails to separate. Children born
with the extra body chromosome have 47, rather than the normal 46, chromosomes.
Individuals with Down syndrome have certain recognizable facial and body features.
Heavier eyelids, shorter noses, enlarged tongues and shorter statures are some of the
physical effects that are usually combined with a degree of mental retardation. A
lower resistance to diseases frequently reduces life expectancies.
Klinefelter Syndrome
Nondisjunction of the sex chromosomes could produce some males with an extra X
chromosome (XXY). Although male, these individuals have some secondary feminine
traits, as well as experiencing possible sterility and lower than normal intelligence.
Turner Syndrome
Nondisjunction of sex chromosomes could also produce X0 females, where an X
chromosome is missing. Such persons are shorter, stockier and have enlarged
necks. They also have incomplete development of sexual organs and are therefore
usually sterile.
Phenylketonuria (PKU)
A recessive gene in homozygous condition causes Phenylketonuria. The gene is found
on a autosomal chromosome. Both parents would be heterozygous, or carriers.
A person who is homozygous for this condition cannot produce an enzyme necessary
to break down an amino acid called phenylalanine, which is common in many foods.
Incomplete breakdown of this amino acid leads to the buildup of a substance called
phenylpyruvic acid. An over concentration of this acid results in brain damage and
retardation. Tests on newborn infants can now detect PKU. If an infant is positive,
steps can be taken to assure a more normal development. Presently, such children
follow carefully controlled diets (omitting foods with phenylalanine) until their brain
development is complete. As adults, they can then follow more normal diets.
Biology 30
215
Lesson 13
Tay-Sachs Disease
This disorder is more commonly found in Ashkenazic Jews from Eastern Europe and
in one in 400 000 births in the general population. It is a disorder of the central
nervous system. The gene that produces an enzyme to break down a lipid produced
and stored in tissues of the central nervous system is absent. The lipid accumulates
in the cells and can eventually destroy the nerve cells. The genotype of a person with
Tay-Sachs is homozygous recessive. Carriers of the disorder seem to be unaffected.
Cystic fibrosis
This is the most common genetic disorder found in Canada. It affects 1 in 2000
children. It is thought that 1 in 20 people carry the defect in a heterozygous
condition. Cystic fibrosis causes the secretion of sticky mucus that can block air
passages and create digestive problems. The gene responsible for cystic fibrosis was
located by a group of Toronto doctors, Drs. Lap-Chee Tsui, Frank Collins, and Jack
Riordin working with a research group from Michigan. With this discovery carriers
can be identified. Will it be possible to stop this gene from acting?
Genetic Checks and Genetic Counselling
The rapid development of new scientific techniques now enables doctors, geneticists
and researchers to carry out studies that allow insights into the genotypes and
phenotypes of offspring before they are even born.
Amniocentesis, previously described, is a procedure whereby a doctor can remove
some amniotic fluid from a pregnant female for microscopic examination. This fluid
contains some discarded embryonic cells that can be used for examination of the
chromosomes in a prepared karyotype. Analysis of the proteins in the fluid itself can
also alert observers to any possible disorders that could develop during pregnancy.
A chorionic villi biopsy offers an examination of the chromosomes just as
amniocentesis does, but much sooner. The technique of developing micrographs is
closely coupled with amniocentesis, chorionic villi biopsies and other procedures,
where embryonic cells can be obtained for microscopic examinations. This allows
special equipment to take photographs of chromosomes, particularly in the stages of
meiosis or mitosis when they are in homologous pairings and most clearly visible.
Micrographs can then be cut apart and rearranged with homologous pairings from
the largest to smallest sizes. These are called karyotypes and enable observers to
carry out more detailed studies and faster identifications of any possible
abnormalities.
Using ultrasound and special screens, doctors can also form images of an embryo
still carried within the uterus of its mother. Such images can reveal possible
physical abnormalities.
Some of the techniques used in making embryo analyses have drawn criticisms from
some sources. Particular testing methods could be harmful, if improperly done, or
Biology 30
216
Lesson 13
potentially harmful (ultrasound) until further studies are made. On the other hand,
there are some definite benefits.
Some benefits are:


Identifying certain disorders, such as phenylketonuria, can lead to treatments to
prevent actual expressions of those disorders.
Identified genotypes of parents can also be used for counselling purposes.
Doctors or geneticists can advise potential parents regarding the probabilities of
having children exhibiting certain traits. Based upon such information and
advice, many potential parents can make better informed decisions as to whether
or not to attempt to have their own children, and the possible implications if they
do.
What would genetic counselling involve?
Couples may seek to have professional counselling to determine the possibilities of a
genetic disorder showing up in their future offspring. A genetic counsellor would
begin by building a family history of both families. He/she may develop pedigrees, do
a biochemical blood analysis, and do a karyotype. When all of the tests are done and
an analysis of them is completed, then the counsellor would discuss with the couple
the possibilities of a genetic disorder showing up in their offspring.
Summary
Heredity and genetics and their applications have likely concerned humans from the
time that plants and animals were domesticated and subjected to controlled
breeding. It was probably only in the last 50 years, however, that major advances
have taken place to the point where humans can use techniques to significantly alter
or control gene processes and genotypes. Plant growers and livestock producers are
presently receiving the benefits of research that are greatly improving strains and
varieties. With genetic engineering or manipulation (employing such techniques as
chromosome or gene transpositions between different organisms), there is a
tremendous potential for creating modified or new chromosome and gene
combinations. As this happens, new organisms or varieties will be introduced with
characteristics once thought to be impossible to attain.
Biology 30
217
Lesson 13
As genetic knowledge accumulates and new techniques and applications are
implemented, questions will continue to arise concerning our own involvements. No
doubt some of the more important concerns will be directed at the degree to which
human manipulations should proceed. To what extent should "natural" processes of
breeding and selection be replaced by controlled breedings, selections and genetic
manipulations? Applying such gene or chromosome manipulations more directly to
humans themselves could make such questions even more critical. How those
questions are resolved could very well determine what sorts of relationships occur
among humans and the kinds of changes that will take place in future generations
Biology 30
218
Lesson 13
Additional Information for doing pedigrees and sexlinkage questions.
Pedigrees, Sex-linkage Helps
Pedigrees show the history for the inheritance of a particular trait throughout several
generations of a family. Whether the trait is an inherited disorder or following
through the inheritance of ‘bent little fingers’ in a family, a pedigree is an organized
way to lay out and analyze the genetic information.
Additional information about pedigrees:
1. In a pedigree, males are symbolized as squares
and females the circles
.
The horizontal lines show mating partners while the vertical lines indicate the
offspring of the parents.
On a chart, a generation is commonly represented by a Roman numeral. The
following illustration shows four generations. Specific individuals are identified by
common (or Arabic) numerals. To identify a particular individual on a chart, a
generation number is followed by an Arabic number. The individual pointed out
by the arrow in the following illustration is identified as III-7. Individuals II-2, II-5
and IV-1 display a particular trait being studied.
2. An individual having the disorder is shown by a darkened square or circle. If an
individual is a carrier the square or circle is half shaded in. Sometimes the
carriers are not shown. It is up to you to determine which are carriers.
Biology 30
219
Lesson 13
3. To determine the inheritance pattern in a pedigree for humans you must:



Assess the family members above and below the individual that has the
disorder attempt to determine their genotypes.
Check to see whether the trait has ‘skipped’ a generation. Often this is an
indicator of a recessive inheritance. A dominant trait usually doesn’t skip
a generation.
It is a good idea to check back to the dominant recessive crosses
(assign.13) to see:
 the possible outcomes if both parents are homozygous for the trait
in a cross or
 what the outcome will be if the parents are both heterozygous (F1
cross).
 Another option would be to see what the outcome would be if one
parent is homozygous for the trait and the other is heterozygous.
(These are basic patterns that true for any dominant – recessive cross. Knowing
them will aid you in identifying an inheritance pattern of a pedigree as well as
helping you in the multiple-choice questions).
4. Check to see if the inheritance pattern is sex linkage. (See below)
To identify a sex-linked trait:


More males than females have recessive sex-linked disorders.
Pick out a female who does show the trait.
Then:
1. Her father must show the trait
2. All her male offspring MUST show the trait.
The sex chromosome combinations are XX for female and XY for male. The Y
chromosome determines maleness. It is small and carries few genes (none for the
purposes of grade 12 biology). The X on the other hand, is large and carries many
essential genes. If any trait appears on the X, the Y usually can’t cover it up so it
expresses itself in a male. In a female, the combination of XX means that if one X
carries '‘recessive ' trait, the ‘normal’ (dominant) X would mask it. Because of this,
females are less likely to show sex-linked traits.
A female will show a recessive trait if both X-chromosomes are carrying it. This
means that one of the recessive genes came from her father. He has to show the trait
because his other chromosome is an Y chromosome (it doesn’t carry the gene).
Sons receive their X chromosome only from their mothers and their Y, only from their
father. If the mother has the trait on both X’s, the sons will show the trait.
Daughters only receive their father’s X chromosome.
Try the practice Questions
Biology 30
220
Lesson 13
Practice Questions:
1.
Albinism is an inability to make the pigment melanin. An albino has white hair
and pink eyes. A person with normal pigmentation has a genotype of AA or Aa.
An albino has a genotype of aa. Answer the questions about the following
pedigree.
a.
b.
2.
What is the genotype of number 2?
What are the genotypes of numbers 3, 4, 5, and 6?
A child has a blood type of AB and the mother has a blood type of A.
a.
b.
What are the possible genotypes of the father?
Could the father be type O? Explain.
3.
In cats the genotype BB is black, Bb is tortoise shell and bb is yellow. The
gene is on the X chromosome. A tortoise shell female is crossed with a black
male. What offspring would be expected? Would you expect to find any
tortoise shell males?
4.
A color-blind man marries a woman with normal vision. Five offspring result.
One of the male children is colour-blind.
a.
b.
Biology 30
What is the genotype of the mother? Explain how you know this.
Is it possible for this couple to have a colour-blind daughter? Explain.
221
Lesson 13
5.
A female carrier of haemophilia does not have the disease. Her genotype is
X S X s . S is the gene for normal blood clotting, s for haemophilia. The carrier
female mates with a normal male.
a.
b.
What percent of the female offspring are normal, and what percent are
carriers?
What percent of the male offspring are normal, and what percent are
haemophilic?
Questions 6-8 are based on the following pedigree.
Myopia (nearsightedness)
6.
What inheritance pattern is indicated by the pedigree chart showing
myopia in a particular family? (Inheritance pattern could refer to such
combinations as dominant-recessive, codominance, incomplete
dominance, sex-linkage ...)
Inheritance pattern: _______________________________________________
7.
Biology 30
What is your reasoning in the selection of the answer for 1?
222
Lesson 13
8.
a.
Establish the letters that you wish to use to express normal vision
and myopia.
Normal vision: ____________________
Myopia: ____________________
b.
c.
(3)
Biology 30
Using the letters you have chosen in (a), indicate the probable
genotypes of the following individuals:
I-1
____________________
I-2
____________________
II-1
____________________
II-2
____________________
II-3
____________________
II-5
____________________
IV-1 marries a woman who is normal-visioned and whose family
never had any recorded instances of myopia for many generations.
i.
This couple should not have any myopic children. ________
This couple could have myopic children.
_______
(Place a checkmark beside the more accurate statement.)
ii.
Give a reason for your choice in i.
223
Lesson 13
Practice Question Answer Sheet
1.
2.
3.
a.
The genotype of number 2 is Aa. The genotype has to be this as number
1 shows the trait so has the genotype aa. Two of the offspring do not
have the trait so the dominant gene had to come from the mother.
b.
The genotype
a.
The possible genotypes of the father are I B I B or I Bi .
b.
The father could not be type O because the child is I A I B . The IB has to
come from the father.
number 3 is
number 4 is
number 5 is
aa
Aa
Aa
The easiest way to find out is to do the Punnett Square.
The cross is
25% are black females
XB XB
25% are black males X B Y



25% are tortoise shell females


25% are yellow males X b Y

You would not expect to find tortoise shelled males because you need the Bb
combination to get the tortoise shell colour and you cannot have both as the
male only has one X.
Biology 30
224
Lesson 13
4.
a.
The genotype of the mother is X C X c . The reason we know this is
because she has a male child that is colour-blind. The male offspring get
their X from their mother. She has normal vision which means she has
one X C and of course that does not cause colour-blindness. To pass on
the colour-blind trait means she had to be a carrier of the trait which
means she had one X c to give her son.
b.
You may be able to do this question by just looking at it, but I would
recommend that you do all the work “just in case” – then you know that
you are right!! The parent cross was X C Y  X C X c . Do the Punnett
Square to determine if they could have a colour-blind daughter.
Yes it is possible to have a colour-blind daughter.
5.
Again work this question through – do the Punnett Square.
Female carrier’s genotype is X S X s .
The cross is
XS Xs  XS Y
a.
All females are normal with 50% of them being carriers.
b.
50% of the males offspring are normal and 50% are haemophilic.
Biology 30
225
Lesson 13
Answers to Pedigree Chart Showing Myopia
6.
The inheritance pattern is dominant – recessive.
7.
The trait is hidden in parents 1,2 and 3,4 but shows up in their offspring.
(Check out 12 again plus the section on sex-linkage.)
8.
a.
Normal vision:
myopia
b.
I-1 = Nn
I-2 = Nn
II-1 = Nn
II-2 = Nn
II-3 = Nn or NN
II-5 = nn
c.
i.
This couple should not have any myopic children.
ii.
The woman’s family has not had any myopia for many generations
so she is likely NN and even if IV-1 is Nn, myopic children are not.
Biology 30
N
n
226
Lesson 13