General Biology (Bio107)
Chapter 9 – Genetics
• Genetics is the scientific study of heredity or inheritance.
• Every day we observe variations of heritable traits,
e.g. eyes of brown, green, blue, or gray) among
individuals in a population.
• These traits are transmitted from parents to offspring.
• One proposed mechanism for this transmission is the
“blending” hypothesis (J.B. Lamarck).
• This hypothesis proposes that the genetic material
contributed by each parent mixes in a manner
analogous to the way blue and yellow paints blend
to make green.
• Over many generations, a freely mating population
should give rise to a uniform population of individuals.
Offspring resemble their parents more than they do less
closely related individuals of the same species
fw-wt
hw-wt
fw
hw
fw
hw
Variation in the forewings and hind wings of the butter fly species Bicyclus
anynana
- First two photos on the left are of the wild type species
- Others show the natural variation within the "normal" version of the same species.
• However, the “blending” hypothesis appears
incorrect as everyday observations and the results
of breeding experiments contradict its predictions.
• An alternative model, “particulate” inheritance,
proposes that parents pass on discrete heritable
units - genes - that retain their separate identities in
offspring.
• Genes can be sorted and passed on, generation after
generation, in undiluted form.
• Modern genetics began about 150 years ago in an
abbey garden, where a monk named Gregor Mendel
was the first to study and document the particulate
mechanism of inheritance.
• Grew up on a small farm in what
is today the Czech Republic
• Enters an Augustinian monastery
in 1843
• Studies at the University of
Vienna from 1851 to 1853
- becomes influenced by a
physicist who encouraged
experimentation and application
of mathematics to science and a
botanist who aroused Mendel’s
interest in the causes of
variation in plants.
• Teaches at the Brunn Modern
School and lives in the local
monastery with long tradition of
interest in the breeding of plants,
including peas.
• Begins breeding garden peas to
study inheritance around 1857
Johann Gregor Mendel
(1822 – 1884)
Augustinian monk,
teacher & scientist
Old Brno monastery with Mendel’s green house
where he conducted his famous breeding experiments in the
1850s with the common garden pea
• Mendel’s experiments mark the beginning of the era of modern
genetics
• The garden pea Mendel chose to conduct his breeding experiments
had a series of favorable features largely responsible for his
experimental success:
1. Easy to grow and available in many distinguishable varieties
 e.g. blossom color, seed shape, pod color, etc.
2. Strict control over plant matings by switching between the natural
process of self-fertilization and the experimental procedure of
cross fertilization
3. Able to select from a series of easy-to-follow, heritable plant
characteristics
4. Ability to generate true-breeding varieties by performing famous
breeding experiments to generate hybrids
Hybrids are the offspring of two different, true-inbreed varieties
1. Many easily distinguishable heritable traits in the garden pea
2. Strict control over plant matings
In nature, pea
plants typically
self-fertilize,
fertilizing ova
with their own
sperm.
However, Mendel
could also
move pollen from
one plant
to another to
cross-pollinate
plants.
Mendel’s Mono-Hybrid Cross Experiments
• First Mendel cross-pollinated
(hybridized) two contrasting,
true-breeding pea varieties.
• The true-breeding parents are the
P generation and their hybrid
offspring are the F1 generation.
• He then allowed the F1 hybrids
to self-pollinate to produce an
F2 generation.
• It was mainly Mendel’s
quantitative analysis of F2 plants
that revealed the two fundamental
principles of heredity:
1. The law of segregation &
2. The law of independent assortment.
When Mendel allowed the F1 plants
to self-fertilize, the F2 generation
included both purple-flowered and
white-flowered plants.
The white trait, absent in
the F1, reappeared in the
F2 generation
Based on a large
sample size, Mendel
recorded 705
purple-flowered F2
plants and 224
white-flowered F2
plants from the
original cross.
Mendel reasoned that the heritable
factor for white flowers was present
in the F1 plants, but it did not affect
flower color.
Purple flower is a dominant trait
and
white flower is a recessive trait.
The reappearance of white-flowered
plants in the F2 generation
indicated that the heritable factor
for the white trait was not diluted or
“blended” by coexisting with the
purple-flower factor in F1 hybrids.
Mendel found similar 3 to 1 ratios of two traits among
F2 offspring when he conducted crosses for six other
heritable characteristics, each represented by two
different varieties.
For example, after crossing
two true-breeding varieties,
one with round seeds, the
other with wrinkled seeds.
All the F1 offspring had
round seeds, but among
the F2 plants, 75% of the
seeds were round and
25% were wrinkled.
P
F1
• Mendel developed a hypothesis to explain these
results that consisted of four related ideas.
1. Alternative version of genes (different alleles)
account for variations in inherited characters.
– Different alleles vary somewhat in the sequence of
nucleotides at the specific locus of a gene.
• The purple-flower
allele and white-flower
allele are two DNA
variations at the
flower-color locus.
2. For each character, an organism inherits two
alleles, one from each parent.
– A diploid organism inherits one set of chromosomes from
each parent.
– Each diploid organism has a pair of homologous
chromosomes and therefore two copies of each locus.
• These homologous loci may be identical, as in the truebreeding plants of the P generation.
• Alternatively, the two alleles may differ
– In the flower-color example,
the F1 plants inherited a purpleflower allele from one parent
and a white-flower allele from
the other.
maternal
paternal
Chromosomes
3. If two alleles differ, then one, the dominant allele
“A”), is fully expressed in the organism’s appearance.
The other, the recessive allele (“a”), has no noticeable
effect on the organism’s appearance.
4. The two alleles for each character segregate
(separate) during gamete production.
• This segregation of alleles corresponds to the
distribution of homologous chromosomes to different
gametes in meiosis.
– If an organism has identical allele for a particular character,
then that allele exists as a single copy in all gametes.
– If different alleles are present, then 50% of the gametes will
receive one allele and 50% will receive the other.
• The separation of alleles
into separate gametes is
summarized as
Mendel’s law of
allele segregation.
Mendel’s law of segregation
accounts for the 3:1 ratio that
he observed in the F2 generation.
F1 hybrids will produce two classes
of gametes, 50% with the purpleflower allele and 50% with whiteflower allele.
During self-pollination and
fertilization, gametes unite
randomly.
This can produce four equally
likely combinations of sperm
and ovum.
• Genetics has some unique, useful vocabulary.
• An organism with two identical alleles for a character is
homozygous for that character.
• Organisms with two different alleles for a character is
heterozygous for that character.
• A description of an organism’s traits is its phenotype.
• A description of its genetic makeup (‘allele
combination”) is referred to as its genotype.
– Two organisms can have the same phenotype but
have different genotypes if one is homozygous
dominant and the other is heterozygous.
Test Cross
It is not possible to predict
the genotype of an organism
with a dominant phenotype.
The organism must have
one dominant allele, but it
could be homozygous
dominant or heterozygous.
A test cross = breeding of
homozygous recessive, with
dominant phenotype,
but unknown genotype
Test cross can determine the
identity of the unknown allele.
Dihybrid Cross Experiments
In dihybrid cross experiments, Mendel
studied the inheritance of seed color
and seed shape.
- allele for yellow seeds (Y) is
dominant to the allele for green
seeds (y).
- allele for round seeds (R) is
dominant to the allele for
wrinkled seeds (r).
Mendel crossed true-breeding plants
that had yellow, round seeds (YYRR)
with true-breeding plants that has
green, wrinkled seeds (yyrr).
100% of the F1 plants had yellow,
round seeds
One possibility is that the two
characters are transmitted from
parents to offspring as a package,
i.e. the Y and R alleles and y and r
alleles stay together 9”are linked”)
If this were the case, the F1
offspring would produce
yellow, round seeds.
The F2 offspring would
produce two phenotypes
in a 3:1 ratio, just like a
monohybrid cross.
However, this hypothesis was not
consistent with the results Mendel
observed .
Dihybrid Cross Outcome
An alternative hypothesis is that the
two pairs of alleles segregate
independently of each other.
The presence of one specific
allele for one trait has no
impact on the presence of a
specific allele for the second
trait.
The F1 dihybrid plants produce
four classes of gametes (YR, Yr,
yR, and yr) in equal amounts.
Law of Independent Assortment
Mendel repeated the dihybrid cross experiment for other pairs of
characters and always observed a 9:3:3:1 phenotypic ratio for the
phenotypes in the F2 generation.
Each character appeared to be inherited independently.
The independent assortment of each pair of alleles during gamete
formation is now called Mendel’s law of independent assortment.
One other aspect that you can notice in the dihybrid cross
experiment is that if you follow just one character, you will observe
a 3:1 F2 ratio for each, just as if this were a monohybrid cross.
Dominant-Recessive Traits in Humans
Human cells like the cells of the garden pea have their genetic
information encoded on genes which are packed into
chromosomes.
Human cells also produce haploid germ cells (= sperm and oocytes)
by meiosis and reproduce by fertilization.
Therefore, even though human cells contain many more genes
( around 100,000 !!) than the garden pea, many heritable traits in
humans follows the same Mendelian principles of inheritance.
Many human traits are known, which are determined by
simple dominant-recessive inheritance at one gene locus.
Allele
Trait
dominant
recessive
brown
blue
earlobe
free lobes
attached
freckles
yes
no freckles
able to taste
not able to taste
six
five
Widow’s peak
straight line
eye color
PTC paper
finger number
hairline
Autosomal recessive human disorders
1. Hereditary deafness
2. Cystic fibrosis (CF)
• The most common lethal genetic disease in the US.
Normal allele codes for a membrane protein that transports
Cl- between cells and the environment.
• If these channels are defective or absent, there are abnormally
high extracellular levels of chloride that causes the mucus coats
of certain cells to become thicker and stickier than normal.
• This mucus build-up in the pancreas, lungs, digestive tract, and
elsewhere favors bacterial infections.
• Without treatment, affected children die before year five.
• The responsible allele is differently distributed amongst different
ethnic groups. One in 25 whites is a carrier of CF.
Probability p
1/1800
1/17000
1/90000
Caucasian Americans births
African American births
Asian American births
• Homozygous individual (= with 2 copies of the defect CF allele)
develops the CF.
Autosomal recessive human disorders
3. Phenylketonuria (PKU)
• This recessively inherited human disorder occurs in 1/10,000 to
1/15,000 births.
• Individuals with this disorder have a lack or a dysfunctional enzyme
due to a mutation in the gene coding for this enzyme.
As a consequence the amino acid phenylalanine and its derivative
phenypyruvate accumulate in the blood to toxic levels.
This leads to serious mental retardation of the affected new born.
• If the disorder is detected, a special diet low in phenyalalanine
usually promotes normal development.
4. Sickle cell anemia
• Homozygous persons have sickled red blood cells, which cause
serious damage to many tissues in the body.
• Disorder affects 1/500 births of African-Americans; and 1/10
African Americans is a heterozygote carrier of the sickle cell allele.
It is a very rare disease amongst other ethnic groups.
5. Albinism
• People or mammals affected by this rare autosomal recessive
disorder lack the UV light absorbing and DNA protecting pigment
melanin in their skin, hairs and eyes.
• Their skin or fur therefore appears white and the iris of their eyes
appears red colored.
• Affected persons or mammals (= albinos) are very easily
sunburned and have a high risk of developing forms of skin cancer.
Many reasons account for the manifestation of a recessive genetic
disorder within a human population
1. Prolonged geographic isolation and inbreeding
There is an increased probability that a recessive allele breaks
through, i.e. it becomes dominant in an homozygous individual
if close relatives marry and have children (consanguinity).
The manifestation of recessive disorders can frequently be observed
in small societies which were geographically isolated for extended
periods of times (e.g. on an island, in a hard-to-reach valley, etc.).
Hereditary deafness is common amongst certain families living on
Martha’s Vineyard in Massachusetts.
2. Co-evolutionary aspects & Adaptive reasons
The allele for sickle cell anemia manifested itself in a high percentage
within the African American population.
It gives its carriers a certain advantage/protection against malaria
infection.
Carriers of the sickle cell anemia allele in Africa are less likely
affected by malaria-causing Plasmodium strains.
Autosomal dominant human disorders
Dominant inherited human disorders are serious disorders caused
by a dominant allele. Only one (dominant) allele is required to lead
to the expressed phenotype, i.e. the human health-affecting
disorder.
Examples of prominent dominant disorders in humans are:
1. Familial polydactyly
2. Achondroplasia
3. Alzheimer’s disease (AD)
1. Familial Polydactyly
• Individuals are born with extra fingers or toes.
It is due to an allele dominant to the recessive allele for five
digits per appendage.
• 399 individuals out of 400 have five digits per appendage.
2. Achondroplasia
• Autosomal dominant inherited or
acquired (80% of all cases) genetic
disorder which is responsible for
most common form of dwarfism in
humans.
• Characterized by the appearance of
short stature with disproportionately
short arms, short legs, a large head
with pronounced forehead due to
impaired cartilage formation.
• Connection to mutations in the gene
coding for the receptor (FGFR) of
the growth hormone
fibroblast growth factor
(FGF).
• A lethal dominant allele can escape elimination if it
causes death at a relatively advanced age, after the
individual has already passed on the lethal allele to
his or her children.
• One example is Huntington’s disease, a fatal
degenerative disease of the nervous system.
• The dominant lethal allele has no obvious
phenotypic effect until an individuals is about 35
to 45 years old.
• Lethal dominant alleles are much less common than
lethal recessives because if a lethal dominant kills an
offspring before it can mature and reproduce, the
allele will not be passed on to future generations.
• Genetic disorders are not evenly distributed among all groups of
humans.
• For example:
1. Cystic fibrosis (CF)
 high prevalence (1/2,500) in Jews of eastern
European descent
2. Tay-Sachs disease
 affects 1/3,600 births of Ashkenazic Jews
 about 100 times greater than the incidence among nonJews or Mediterranean (Sephardic) Jews
3. Sickle cell anemia
 affects one of 400 African Americans
 caused by the substitution of a single amino acid in
hemoglobin
• Results from different genetic histories of the world’s people
during times when populations were more geographically (and
genetically) isolated.
Human genetics & Co-evolutionary aspects
• The high frequency of heterozygotes with the sickle-cell anemia trait
is unusual for an allele with severe detrimental effects in
homozygotes.
• Interestingly, individuals with one sickle-cell allele have increased
resistance to malaria, a parasite that spends part of its life cycle in
red blood cells.
• In tropical Africa, where malaria is common, the sickle-cell allele
is both a boon and a bane.
• Homozygous normal individuals die of malaria, homozygous
recessive individuals die of sickle-cell disease, and carriers are
relatively free of both.
• Its relatively high frequency in African Americans is a vestige of
their African roots.
Pedigree Analysis
• Rather than manipulate mating patterns of people,
geneticists analyze the results of matings that have
already occurred.
• In a pedigree analysis, information about the
presence/absence of a particular phenotypic trait is
collected from as many individuals in a family as
possible and across generations.
• The distribution of these characters is then mapped
on the family tree.
• For example, if an individual in the third generation lacks a
widow’s peak, but both her parents have widow’s peaks,
then her parents must be heterozygous for that gene
• If some siblings in the second generation lack a widow’
peak and one of the grandparents (first generation) also
lacks one, then we know the other grandparent must be
heterozygous and we can determine the genotype of almost
all other individuals.
Hypothetical family pedigree showing inheritance of the albino trait
female
female
male
male
I.
Genotype
:
1
2
3
4
Aa
Aa
AA
aa
II.
Genotype:
1
2
3
4
5
6
7
Aa
Aa
8
AA
AA or Aa
aa
Aa
Aa
2
3
4
5
aa
AA or Aa
AA or Aa
AA
III.
1
Genotype:
AA or Aa
AA or Aa
Graphic©E.Schmid/2004
Red = family members with albinism
• While some diseases are inherited in a simple Mendelian fashion
due to alleles at a single locus, many other human disorders have
a multifactorial basis.
• These have a genetic component plus a significant
environmental influence.
• Multifactorial disorders include:
Heart disease, diabetes, cancer, alcoholism, and certain
mental illnesses, such a schizophrenia and manic-depressive
disorder.
• The genetic component is typically polygenic.
• At present, little is understood about the genetic contribution to
most multifactorial diseases
• The best public health strategy is education about the
environmental factors and healthy behavior.
Genetic Testing Methods
• A preventative approach to simple Mendelian disorders is
sometimes possible.
• The risk that a particular genetic disorder will occur can
sometimes be assessed before a child is conceived or early in
pregnancy.
• Many hospitals have genetic counselors to provide information to
prospective parents who are concerned about a family history of a
specific disease.
• Important genetic tests are:
1. Amniocentesis
2. Chorionic villi sampling (CVS)
3. Ultrasonic
4. FISH and PCR
1. Amniocentesis
• Can be used at beginning of the 14th to 16th week of pregnancy
to assess the presence of a specific disease.
• Fetal cells extracted from amniotic fluid are cultured and
karyotyped to identify some disorders.
• Other disorders can be identified from chemicals in the
amniotic fluids.
2. Chorionic Villi Sampling (CVS)
• Can allow faster karyotyping and can be performed as early as
the eighth to tenth week of pregnancy.
• This technique extracts a sample of fetal tissue from the
chrionic villi of the placenta.
• This technique is not suitable for tests requiring amniotic
fluid.
3. DNA-based, genetic tests
• Tests such as PCR, RFLP and FISH, detect genetic mutations or
chromosomal defects at birth; many of these are now routinely
performed in hospitals.
• One test can detect the presence of a recessively inherited
disorder, phenyketonuria (PKU).
• This disorder occurs in one in 10,000 to 15,000 births.
• Individuals with this disorder accumulate the amino acid
phenylalanine and its derivative phenypyruvate in the blood
to toxic levels.
• This leads to mental retardation.
• If the disorder is detected, a special diet low in
phenyalalanine usually promotes normal development.
Incomplete Dominance
• In the 20th century, geneticists have extended Mendelian
principles not only to diverse organisms, but also to patterns of
inheritance more complex than Mendel described.
• Not all heritable traits behave in a “classical” dominant-recessive
pattern
• In fact, Mendel had the good fortune to choose a system that was
relatively simple genetically.
• Each character (but one) is controlled by a single gene.
• Each gene has only two alleles, one of which is completely
dominant to the other.
• A clear example of incomplete dominance is seen in the
inheritance of the petal color of snapdragons.
• A cross between a
white-flowered plant
and a red-flowered
plant will produce all
pink F1 offspring.
• Self-pollination of the
F1 offspring produces
25% white, 25% red,
and 50% pink
offspring.
Incomplete Dominance
e.g. Heritable trait = petal color of snapdragon plants
Petal color in snap
P
dragons
Cross of two different,
pure-bred varieties
leads to offspring
which show a blending
of the inbred phenotypes
RR
rr
F1
F2
R
R
RR
Rr
r
Rr
rr
Genotype
X
R
r
Rr
Sperm
Eggs
Phenotype
X
r
Punnett Square
Genotypes
F2
25%
50%
25%
Phenotypes
Graphics©E.Schmid/2002
• While heterozygous F1 offspring of Mendel’s pea
crosses always looked like one of the parental varieties,
heterozygotes in incomplete dominance show a distinct
intermediate (”blended’) phenotype, not seen in
homozygotes
• However, this is not blended inheritance because the
traits are separable (particulate) as seen in the F2
generational plants and in further crosses.
• Offspring of a cross between heterozygotes will show
three phenotypes: both parentals and the heterozygotes
• Incomplete dominance does NOT violate the Mendelian
laws
Phenotype
Familial Hyper
cholesterolemia (fHC)
= Plasma Cholesterol
Concentration
800
(mg/dl)
Clinical manifestations/
symptoms:
High LDL and
cholesterol levels in blood
plasma.
Early arteriosclerosis
(usually before age 40).
Xanthomas in skin and
tendons.
Reduced life expectancy.
1000
600
400
200
0
FH
Homozygous
FH
Heterozygous
Normal
-/-
-/+
+/+
Genotype
Co-Dominance
• Most genes have more than two alleles in a population.
• For example, the ABO blood groups in humans are
determined by three alleles, IA, IB, and I.
• Both the IA and IB alleles are dominant to the i allele
• The IA and IB alleles are codominant to each other.
• Because each individual carries two alleles, there are six
possible genotypes and four possible blood types.
Codominance
More than two alleles responsible for one characteristic (or heritable
trait).
In some cases the genotype does not always dictate the phenotype
in a classical Mendelian pattern.
Classical example is the inheritance of blood types in humans.
The ABO blood groups in humans are determined by not only two but
three alleles.
ALLELE
FOR SIMPLICITY WE
CALL THESE:
IA
A
IB
B
I
O
alleles
IA
i
IB
gene expression
Chromosome
#9
(9q34.1)
no enzyme
“A” enzyme
“B” enzyme
Sugar residues
Pr
“H” enzyme  all humans have it!
“A” enzyme
“B” enzyme
Pr
Pr
“A”
antigen
A
“B”
antigen
B
“A” + “B”
antigen
AB
“O”
antigen
O
Blood type
Graphics©E.Schmid/2002
“Universal acceptor”
Pleiotropy
• Not all genes 9and its allele versions) affect only one
phenotypic character.
• Most genes are rather pleiotropic, which means that they are
affecting more than one phenotypic character.
• For example, the wide-ranging symptoms of sickle-cell
disease are due to a single gene.
• Considering the intricate molecular and cellular interactions
responsible for an organism’s development, it is not surprising
that a gene can affect a number of an organism’s characteristics.
Polygenic Inheritance
• Some characters do not fit the “either-or” basis that Mendel
studied.
• Quantitative characters vary in a population rather along a
continuum.
• These are usually due to polygenic inheritance, the additive
effects of two or more genes on a single phenotypic character.
• For example, skin color in humans is controlled by at least
three different genes.
• Imagine that each gene has two alleles, one light and one
dark, that demonstrate incomplete dominance.
• An AABBCC individual is dark and aabbcc is light.
Polygenic Inheritance
Many characters vary in a population along a continuum due to
the additive effects of two or more genes on a single phenotype.
EXAMPLES OF POLYGENIC INHERITANCE PATTERNS IN HUMANS
ARE:
1. SKIN COLOR
2. HAIR COLOR
3. BODY HEIGHT
• A life forms phenotype depends not only on the impact
of genes but on the influence of environmental
factors as well.
• A single tree has leaves that vary in size, shape, and
greenness, depending on exposure to wind and sun.
• For humans, nutrition influences height, exercise alters
build, sun-tanning darkens the skin, and experience
improves performance on intelligence tests.
• Even identical twins, genetic equals, accumulate phenotypic
differences as a result of their unique experiences.
• The relative importance of genes (“nature”) and the
environment (“nurture”) in influencing human
characteristics is a very old and hotly contested debate.
Gender & X chromosome-linked heritable traits
(X-linked inheritance patterns)
• Although the anatomical and physiological differences between
women and men are numerous, the chromosomal basis of sex is
rather simple.
• In human and other mammals, there are two varieties of sex
chromosomes, X and Y. The human
Y chromosome is much smaller than
the X-chromosome
• An individual who inherits two X chromosomes usually
develops as a female.
• An individual who inherits an X and a Y chromosome usually
develops as a male.
• This X-Y system of mammals is
not the only chromosomal
mechanism of determining sex.
• Other options include the X-0
system, the Z-W system, and the
haplo-diploid system.
• The SRY gene located on the Y chromosome determines male
gender; the generic embryonic gonads are modified into testes.
• In addition to their role in determining sex, the sex chromosomes,
especially the larger X chromosome, have genes for many
characters unrelated to sex.
• These sex-linked genes follow the same pattern of inheritance as
the white-eye locus in Drosophila.
Human Chromosomes & X-linked Inheritance
22 pairs of homologous chromosomes in humans
1
11
2
12
3
13
4
14
15
5
6
16
17
7
18
8
19
9
20 21
10
22
Autosomes
Male
Female
or
X Y
X X
X-linked genes
Sex or gender chromosomes
Male-making genes
e.g. SRY gene
Homologous sections
X
Y
Chromosome
©ESchmid/MesaCollege2001
 In the common fruit fly
Drosophila, the alleles for eye
color and for body color are
located on the X chromosome,
but are not found on the Y
chromosome. Eye color is an
X-linked trait.
 Red eye color allele (W) is
dominant inherited, the white
eye color (w) is the recessive
allele. The tan body color (y+)
is dominant over the yellow
body color (y). Geneticists say,
that red eyes are dominant, Xlinked, while white eyes are Xlinked, recessive inherited.
X-linked Inheritance in Drosophila flies
 While the female parent is homozygous, white eye-colored,
and the male parent is red eyed, the female offspring have red
eye color and the male offspring are hemizygous white eye
colored.
F1 offspring
X-chromosomal inheritance pattern of Hemophilia
reflected in the family pedigree of the European high aristocracy
Queen Victoria
of England
Prince Albert von
Sachsen-CoburgGotha
carrier
Prinz
Heinrich
v. Battenberg
Alice
Heinrich
v. Preussen
Irene
Alexandra
Frederick
Waldemar
Heinrich
Leopold Beatrice
Duke of Albany
Ludwig v. Hessen
Czar Nikolaus
II
Alexis
Alice
Ruprecht
Alfonso
of Spain
Gonzalo
X-linked Inherited Traits in humans
Certain traits and disorders in humans are inherited X-linked.
Examples are;
1. Hemophilia
- “blood coagulation disorder” is due to a mutation of a
gene coding for a cogulation factor, e.g. CF VIII
- mutated coagulation factor protein is defect and not
able to contribute to blood clot formation
- affected individuals (“bleeders’) are permanently bleeding
2. X-linked (“Duchenne-type) muscular dystrophy
- mutated dystrophin gene causes muscle weakening
3. XSCID (X-linked severe combined immunodeficiency
- immune system is defective in “bubble boys”
4. Red-green color blindness

All daughters inherit an X chromosome with the mutation from their father,
and will be carriers; all the sons inherit a normal X chromosome from
their mother and will be phenotypically normal (no bleeders).
X-Inactivation & Barr Body
• Although female mammals inherit two X chromosomes, only
one X chromosome is active.
• Therefore, males and females have the same effective dose (one
copy ) of genes on the X chromosome.
• During early female development,
one X chromosome per cell condenses
into a compact object, a Barr body
which inactivates most of its genes.
FISH
• Selection of X chromosome is random
• Consequentially, females consist of
mosaic of cells
• The condensed Barr body
chromosome is reactivated
in ovarian cells that produce ova.
DAPI
stain
• A mosaic pattern is evident in women who are heterozygous for
a X-linked mutation that prevents the development of sweat
glands. A heterozygous woman has patches of normal skin and
skin patches lacking sweat glands.
• The orange and black fur pattern on cats is due to patches of
cells expressing orange allele while others have non-orange
allele.
Linked Alleles (Genes) & Recombination frequency



Genes on the same chromosome tend to be inherited together (=
linked genes)
Linked genes are genes whose loci are on the same chromosome and
usually close together; they are passed on together and don’t follow
the Mendelian law of independent assortment of genes
First evidence of linked genes was shown by W. Bateson & R. Punnett
while doing crossing experiments with doubly heterozygous sweet
pea plants. They studied two characteristics (= Dihybrid cross)
ALLELE
1. FLOWER COLOR:
recessive red p
dominant purple
P
1. POLLEN SHAPE:
recessive short
dominant long
l
L
P
F1
all (100%) =
PPLL x
PpLl
ppll
Purple-Long
Linked genes/alleles
•The Bateson/Punnett experiment with sweet pea plants (1908)
Two F1 hybrid
Pea plants
Phenotype
X
PpLl
Phenotypes
Purple-Long
Purple-Round
Red-Long
Red-Round
PpLl
Genotype
Predicted
Offspring
(9:3:3:1)
Observed
Offspring
215
71
71
24
284
21
21
55
Recombinants due to
Crossing over events in
Meiosis I
 Discrepancy in the phenotypic ratio is explained by the fact that the
gene coding for the color (P = purple; r = red) and the gene coding
for the pollen shape (L = long; l = round) were located on the same
chromosome; both gene versions were linked genes.
 They were not independently assorted during meiosis; the
observed new phenotypes in F2 can be explained by
recombination events which happened during Prophase I of
meiosis..
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