Transcript Gene Interaction,sex linked inheritance

Gene Interaction, Chromosomal
Theory of inheritance & Sex
Linked inheritance
Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany deptt.
P.G.G.C.G. -11,Chandigarh
Snapdragon’s flower
colour
Incomplete dominance
Ratio ; 1:2:1)
Incomplete dominance
is the phenomenon
where none of the
gene is dominant over
the other.
Codominance
1.
In codominance, both alleles make a
product, producing a combined
phenotype.
2.
Examples :
a. The ABO blood series, in which a
heterozygous IA/IB individual will express
both antigens, resulting in blood type AB.
Multiple alleles (human blood type)
One gene has more than two alleles.
 The human ABO gene has three alleles (IA, IB, i).
 Two alleles (IA, IB) are codominant to each other, and
these two alleles are dominant to the i allele.

Fig. 12.1 Allelic forms of a gene
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Biology of the ABO gene
No of genotypes?
No of phenotypes?
Transfusion rule
Complementary genes (9:7)

Complementary genes are those non
allelic genes,which independently show a
similar effect,but produce a new trait
when present together in dominant form.
Complementary Genes
Epistasis
1. In epistasis, one gene masks the expression of another,
but no new phenotype is produced.
a. A gene that masks another is epistatic.
b. A gene that gets masked is hypostatic.
2. Several possibilities for interaction exist, all producing
modifications in the 9:3:3:1 dihybrid ratio:
a. Epistasis may be caused by recessive alleles, so that a/a
masks the effect of B (recessive epistasis).
b. Epistasis may be caused by a dominant allele, so that A
masks the effect of B. ( Dominant Epistasis)
c. Epistasis may occur in both directions between genes,
requiring both A and B to produce a particular phenotype
(duplicate recessive epistasis).
White: W_
Green: wwyy
Yellow: wwY_
Dominant Epistasis (12:3:1)
•
Allele W is epistatic to Y and y: it suppresses the expression of
these pigment-producing genes.
•
Allele W is dominant because a single copy of the allele is sufficient
to inhibit pigment production.
Recessive epistasis ( 9:3:4 ratio )
Wild mice have individual hairs with an agouti pattern, bands of
black (or brown) and yellow pigment. Agouti hairs are produced by
a dominant allele, A. Mice with genotype a/a do not produce the
yellow bands, and have solid-colored hairs.
b.
The B allele produces black pigment, while b/b mice produce brown
pigment. The A allele is epistatic over B and b, in that it will insert
bands of yellow color between either black or brown.
c.
The C allele is responsible for development of any color at all, and so
it is epistatic over both the agouti (A) and the pigment (B) gene loci. A
mouse with genotype c/c will be albino, regardless of its genotype at
the A and B loci.
d.
In the cross A/a C/c X A/a C/c, the offspring will be:
i. 9⁄16 agouti (A/– C/–).
ii. 3⁄16 solid (a/a C/–).
iii. 4⁄16 albino (3/16 A/– c/c + 1/16 a/a c/c).
Fig. 12.9 Recessive epistasis: Generation of an F2 9:3:4 ratio for coat
color in rodents
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Supplementary genes ( 9:3:4)

Supplementary genes are a pair of non
allelic genes, one of which produce its
effect independently when in dominant
state, while dominant allele of other is
without any independent effect,but is able
to produce a new trait along with the
dominant allele of the former
Duplicate genes (15:1)

Duplicate genes are two or more
independent genes present on different
chromosomes,which determine same or
nearly same phenotype,so that either of
them can produce the same effect.The
independent genes do not show
cumulative effect.
Pleiotropic genes
Pleiotropic genes are those genes which
have multiple effect, because they
influence a number of traits
simultaneously.
 The most evident effect is termed as
Major /primary effect.,while the less
evident effects are known as minor/
secondary effect

1902: Walter Sutton and Theodor Boveri
Chromosome theory of inheritance:
1.
Independently recognized that the transmission of chromosomes
from one generation to the next parallels inheritance of Mendelian
factors.
2.
Mendelian factors (genes) are located on chromosomes.
3.
Support for theory derived from study of sex chromosomes.
Sex linkage
1910: Thomas Hunt Morgan (Nobel Prize 1933)
Experiments with Drosophila demonstrated chromosome
theory of heredity
1.
Discovered a mutant white-eyed male fly (wild type color is red).
2.
Next, crossed wild type female with white-eyed male. All F1
offspring had red eyes (therefore white is recessive).
3.
Crossed F1 x F1  F2 (3,470 red/782 white).
4.
All white-eyed flies also were male.
5.
Morgan hypothesized that eye color gene is located on the X
chromosome.
6.
Ratio of white and red-eyed flies is 1/4.4 (observed) << 1/3
(expected); white-eyed flies have lower viability.
Parental Cross
F1 x F1
Assuming eye color gene is located on the X chromosome:
1.
Males are hemizygous (w/Y) because there is no homologous gene
on the Y chromosome.
2.
Females are either homozygous (w+/w+) or heterozygous (w+/w).
3.
F1 flies were w+/w (females) and w+/Y (males).
F1 x F1
w+ (X)
Y
w+ (X)
w+/w+
XX
w+/Y
XY
w+/w
XX
w/Y
XY
w (X)
Morgan’s hypothesis confirmed by reciprocal crosses:
P Cross
w (X)
Y
w+ (X)
w+/w
XX
w+/Y
XY
w+ (X)
w+/w
XX
w+/Y
XY
P Cross
w+ (X)
Y
w (X)
w/w+
XX
w/Y
XY
w/w+
XX
w/Y
XY
w (X)
100% Red
50%/50%
P Cross
w+ (X)
Y
w (X)
w/w+
XX
w/Y
XY
w (X)
w/w+
XX
w/Y
XY
 F1 of red-eyed females and white-eyed males
X-linked diseases
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X-linked diseases are those for which the gene is
present on the X chromosome.
X-linked diseases show inheritance patterns that
differ from autosomal diseases.
This occurs because males only have one copy of
the X chromosome (plus their Y chromosome) and
females have two X chromosomes.
Because of this, males and females show
different patterns of inheritance and severity of
manifestation. While there are both dominant
and recessive X-linked diseases, there are some
characteristics that are common to X-linked
disorders in general
X-linked genes are never passed from
father to son.
 The Y chromosome is the only sex
chromosome that passes from father
to son.
 Males are never carriers – if they
have a mutated gene on the X
chromosome, it will be expressed.
 Males are termed hemizygous for
genes on the X chromosome.

X-linked recessive
Hereditary pattern in which a recessive
gene on the X chromosome results in the
manifestation of characteristics in male
offspring and a carrier state in female
offspring
 X-linked recessive diseases are those in
which a female must have two copies of
the mutant allele in order for the mutant
phenotype to develop.
 Many X-linked recessive disorders are
well-known, including color blindness,
hemophilia.

The pattern for the pedigree of Xlinked recessive inheritance
Pattern of x linked recessive inheritance
Hemophilia
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The blood fails to clot normally
Lacking a blood clotting factor
VIII(antihemophilic globulin, AHG),IX
bleeding from even minor cuts
in 1,500 newborn males. Most (75%) have
hemophilia A, a lack of clotting factor VIII.
Hemophilia B- "Christmas Disease" is a defect in
clotting factor IX.
Transfusions of fresh whole blood or plasma or
factor concentrates control bleeding
Hemophilia
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Hemophilia is a disease in which the blood does not clot when exposed to air. People
with hemophilia can easily bleed to death from very minor wounds. Hemophilia is
another sex-linked trait.
Hemophilia is treated by injecting the proper clotting proteins, isolated from the
blood of normal people. In the early 1980’s, the blood supply was contaminated by
HIV, the AIDS virus, and many hemophiliacs contracted AIDS at that time.
Queen Victoria of England, who lived through most of the 1800’s, apparently had a
mutation on one of her X chromosomes that caused many of her descendants to
have hemophilia. Most importantly, Alexis, son of the Czar of Russia had it, which
contributed to the Russian Revolution and the rise of communism.
A Pedigree of Hemophilia in the Royal
Families of Europe
Inheritance of hemophilia
Colorblindness
We have 3 color receptors in the
retinas of our eyes. They respond
best to red, green, and blue light.
 Each receptor is made by a gene.
The blue receptor is on an
autosome, while the red and
green receptors are on the X
chromosome (sex-linked).
 Most colorblind people are males,
who have mutated, inactive
versions of either the red or the
green (sometimes both) color
receptors. Most females with a
mutant receptor gene are
heterozygous: the normal version
of the receptor genes gives them
normal color vision.
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Inheritance of Colorblindness
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A heterozygous female has
normal color vision. Sons get
their only X from their mother.
So, ½ of the sons of a
heterozygous mother are
colorblind, and ½ are normal.
A colorblind male will give his
X to his daughters only. If the
mother is homozygous normal,
all of the children will be
normal. However, the
daughters will heterozygous
carriers of the trait, and ½ of
their sons will be colorblind.
Thanks