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Mendelelian
Genetics
Dr. R. Siva
VIT University, INDIA
[email protected]
1
Gregor Mendel
(1822-1884)
Responsible
for the Laws
governing
Inheritance of
Traits
2
1822 - Brunn in Austria, now, Brno
(Czecholovakia)
Financial and Health Problem
Unable to continue studies
Joined St.Augustinian Monastery
1847 – Priest
1851 – Higher studies at Vienna
University
1854- Science teacher
3
Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and
tested some 28,000
pea plants
He found that the
plants' offspring
retained traits of
the parents
4
Gregor Johann Mendel
Austrian monk
Developed the laws
of inheritance
Presented his findings
in the Natural History
Society of Brunn –
1865
Paper entitled,
“Experiments in Plant
Hybridization” – 1866
German language.
5
 Mendel's work was not recognized until the
turn of the 20th century
 1900 - Carl Correns, Hugo deVries, and Erich
von Tschermak rediscover and confirm
 Called the “Father of Genetics“
6
Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
7
Mendel’s workplace
Fig. 2.5
8
Particulate Inheritance
Mendel stated that
physical traits are
inherited as “particles”
Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
9
Genetic Terminology
 Trait - any characteristic that
can be passed from parent to
offspring
 Heredity - passing of traits
from parent to offspring
 Genetics - study of heredity
10
Types of Genetic Crosses
 Monohybrid cross - cross
involving a single trait
e.g. flower color
 Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
11
Designer “Genes”
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
 Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
12
More Terminology
 Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
13
Genotypes
 Homozygous genotype – When the
two alleles are same (dominant or
2 recessive genes) e.g. TT or tt;
also called pure
 Heterozygous genotype – When
the 2 alleles are different- one
dominant & one recessive
allele
(e.g. Tt); also called
hybrid
14
15
Punnett Square
Used to help
solve genetics
problems
16
17
Equation
The formula 2n can be used, where
“n” = the number of heterozygous
traits.
Ex: TtRr, n=2
22 or 4 different kinds of
gametes are possible.
TR, tR, Tr, tr
18
Dihybrid Cross
TtRr X TtRr
Each parent can produce 4 types of
gametes.
TR, Tr, tR, tr
Cross is a 4 X 4 with 16 possible
offspring.
19
RESULTS
9 Tall, Red flowered
3 Tall, white flowered
3 short, Red flowered
1 short, white flowered
Or: 9:3:3:1
20
Genes and Environment
Determine Characteristics
21
Mendel’s Pea Plant
Experiments
22
Why peas, Pisum sativum?
Can be grown in a small
area
Produce lots of offspring
Produce pure plants when
allowed to self-pollinate
several generations
Can be artificially crosspollinated
Bisexual.
Many traits known.
Above all, easy to grow
23
Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
24
Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a paintbrush
He could snip (cut) the
stamens to prevent
self-pollination
He traced traits through
the several generations
25
26
How Mendel Began?
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
27
Eight Pea Plant Traits
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
28
29
30
Mendel’s Experimental Results
31
Did the observed ratio match
the theoretical ratio?
The theoretical or expected ratio of
plants producing round or wrinkled seeds
is 3 round :1 wrinkled
Mendel’s observed ratio was 2.96:1
The discrepancy is due to statistical
error
The larger the sample the more nearly
the results approximate to the
theoretical ratio
32
Generation “Gap”
Parental P1 Generation = the parental
generation in a breeding experiment.
F1 generation = the first-generation
offspring in a breeding experiment. (1st
filial generation)
From breeding individuals from the P1
generation
F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
From breeding individuals from the F1
generation
33
Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
34
Monohybrid
Crosses
35
P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
36
P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
37
F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
38
F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
39
What Do the Peas Look Like?
40
…And Now the Test Cross
Mendel then crossed a pure & a
hybrid from his F2 generation
This is known as an F2 or test
cross
There are two possible
testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
41
F2 Monohybrid Cross
st
(1 )
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
42
F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
43
F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the
offspring have SAME genotype as
parents
44
Practice Your Crosses
Work the P1, F1, and both
F2 Crosses for each of the
other Seven Pea Plant
Traits
45
Mendel’s Laws
1. Law of Dominance
2. Law of Segregation
3. Law of Independent assortment
46
Law of Dominance
States
that
on
crossing
homozygous organisms for single
pair of contrasting characters, only
one characters make its appearance
in F1 generation and is name as
Dominant character.
47
Results of Monohybrid Crosses
Inheritable factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the
other from the father
True-breeding individuals are
homozygous ( both alleles) are the
same
48
Law of Dominance
In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation.
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr (round seeds)
49
Law of Dominance
50
Drosophila
Man
Character
Dominant
Recessive
Body Colour
Eye colour
Gray
Red
Black
White
Colour of hair
Form of hair
Colour of eye
Lips
Dark
Curly
Brown
Broad and
thicken
A,B,AB
Light
Straight
Blue
Thin
Blood group
O
51
.
Law of Segregation
Two allele of a gene remain
separate and do not contaminate
each other in F1 or hybrid.
OR
The two alleles for each trait
separate during gamete formation.
52
Law of Segregation
During the formation of gametes
(eggs or sperm), the two alleles
responsible for a trait separate
from each other.
Alleles for a trait are then
"recombined" at fertilization,
producing the genotype for the
traits of the offspring.
53
Law of segregation
54
Applying the Law of Segregation
55
Law of Independent
Assortment
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
This law can be illustrated using
dihybrid crosses.
56
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
57
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
58
Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
59
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
60
Question:
How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
61
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
62
Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
63
Summary of Mendel’s Hypothesis
1. Genes can have alternate versions called
alleles.
2. Each offspring inherits two alleles, one
from each parent.
3. If the two alleles differ, the dominant
allele is expressed. The recessive allele
remains hidden unless the dominant
allele is absent.
4. The two alleles for each trait separate
during gamete formation.
This now
called: Mendel's Law of Segregation
64
The elegance of mendel’s experiments
was partly due to the complete
consistency between his observation and
Hypotheses he developed.
However, after mendel’s work was
rediscovered, it became clear that
simple
medelian
model
did
not
adequately
predict
experimental
observations in all situations.
65
Variations on Mendel
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance
Multiple Alleles
Epistasis
Polygenic Inheritance
66
Incomplete Dominance
and
Codominance
67
Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two parental
varieties.
Carl Correns Crossed 4’O Clock Plant (flower)
red (RR) x white (rr)
R
R
RR = red flower
rr = white flower
r
r
68
Incomplete Dominance
R
R
r
Rr
Rr
r
Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
69
Incomplete Dominance
70
Another Example
71
Codominance
Both the alleles can be
expressed
Eg. Red cows crossed with
white will generate Roan
cows.
Might seem to support
blending theory.
But F2 generation
demonstrate Mendalian
genetics (1:2:1)
72
Codominance
Two alleles are expressed in heterozygous
individuals.
Example: blood type
1.
2.
3.
4.
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
73
In humans, there are four blood types:
A,B,AB and O
Blood type is controlled by three alleles:
A,B,O
O is recessive, two O alleles must be present
for the person to have type O blood
A and B are Codominant. If a person
receives an A allele and B allele, their
blood type is AB type
Crosses involving blood type often use an I to
denote the alleles
74
Summary of Dominance Relationships
Dominance
X
complete
X
incomplete
X
codominance
75
Try these
In a plant species, if the B allele (blue
flowers) and the b allele is (white
flowers) are incomplete dominant (Bb is
light blue).
What offspring ratio is expected in a
cross between blue flowered with light
blue flowered plant?
What would be the phenotypic ratio of the
flowers produced by a cross between two
light blue flowers?
76
LETHAL GENE
A lethal gene causes death of individual in
the appropriate genotype before they
reach adulthood.
Lethal are generally recessive, resulting in
the death of the recessive homozygote.
- Some lethals are dominant.
In 1904 French geneticist Lucien Cuenot,
discovered a recessive lethal affecting
coat colour in mice.
77
LETHAL GENE
He observed that yellow colour is
dominant;
But while crossing 2 yellow mice, he got
only 2:1 ratio.
While he crossed yellow with recessive
wild type (grey) found that all are
yellow.
Concluded all yellow mice were
heterozygotes.
78
LETHAL GENE
Later it was suggested that homozygosity
for yellow is lethal, and that these
individuals died in utero, a fact
observed by histological studies.
79
Recessive Lethal Alleles
80
Manx Cat
Manx Cat – Tailless cat is
another trait caused by
an allele that has
dominant effect in
heterozygous and is
lethal in homozygotes.
The Manx and normal
alleles are denoted by M
and m respectively
Mm X Mm
81
Other
example
is
achondroplasia, the most
common form of dwarfism,
with a normal length body
trunk but shortened limbs.
82
Many
genes
have
alleles that affect
the
rate
of
mortality but are
not lethals.
In general these are
termed
as
deleterious
or
detrimental alleles.
83
PLEIOTROPY
- Impact of a single gene on
more than one characteristic
- Sickle-cell disease
- Most common inherited
illness among black people
- RBCs are sickle-shaped
Can cause many problems
RBCs: Sickle-cell disease
Normal RBCs
84
Sickle Cell Anemia
Under conditions of low oxygen
tension, hemoglobin S will
precipitate, causing cells to
sickle
Mutations in same amino acid
Some individuals die in
childhood;
Some individuals have mild
symptoms
HbA: V – H – L – T- P – G –G
HbS: V – H – L – T- P – V–G
85
86
Sickle cell anemia may be the result of a
genetic mutation that happened in malariaprone regions like Africa thousands of
years ago.
People with sickle cell trait may have been
more likely to survive malaria epidemics and because they survived when others did
not, this allowed the trait to be passed
down through generations.
87
PKU
88
Gene Interaction and Epistasis
Mendel was lucky that in his studies each
character was governed by a single gene
Typical Dihybrid ratio: 9:3:3:1
The phenomenon of two or more genes
governing the development of single
character is known as Gene interaction.
When one gene affects in anyway the
expression
of
another
gene,
the
phenomenon is called Epistasis.
89
Types of Gene Interaction
1. Typical Dihybrid ratio for a single trait
2. Duplicate gene action
3. Complementary gene action
4. Supplementary gene action
5. Inhibitory gene action
6. Masking gene action
7. Polymeric gene action
And
8. Additive gene action.
90
1.Typical Dihybrid ratio for a single trait
Pea
Rose
Single
Parents:
Walnut
Pea
X
(PPrr)
Ross
(RRpp)
Walnut (PpRr)
91
A typical dihybrid ratio in the case of
single character, Viz Comb shape in
poultry.
Genotype Ratio : 9:3:3:1
Where,
9: Walnut
3: Pea
3:Rose
1: Single
92
2. Duplicate Gene Action:
The non-floating habit
in rice is controlled
by two genes A1/A1;
A2/A2
Like that floating
habit is controlled
by a1/a1; a2/a2
93
Here, the presence of a single dominant
allele of any of the two genes governing
the
trait
produces
the
dominant
phenotype (non-floating). The recessive
genotype (floating habit) is produced
only when the genes are in the
homozygous recessive state. The genes
that show duplicate interaction is called
Duplicate gene action.
94
Duplicate Dominant Eptistasis (15:1)
A;B
A;b
a;B
A;B A/A;B/B A/A;B/b A/a;B/B
a;b
A/a;B/b
A;b
A/A;B/b A/A;b/b A/a;B/b A/a;b/b
a;B
A/a;B/B A/a;B/b a/a;B/B a/a;B/b
a;b
A/a;B/b A/a;b/b a/a;B/b a/a;b/b
Non Floating
Floating
15
:1
95
3. Complementary gene action
In a sweet pea the development of purple
flowers requires the presence of 2 dominant
genes, C and R, e.g., CCRR.
When either C or R alone present purple
flowers cannot be produced; as a result white
flowers are obtained
e.g., ccRR Or CCrr Or ccrr
96
Parents:
CCRR
(Purple)
Gametes:
CR
F1
X
ccrr
(White)
cr
CcRr
(Purple)
97
A;B
A;b
a;B
A;B A/A;B/B A/A;B/b A/a;B/B
a;b
A/a;B/b
A;b
A/A;B/b A/A;b/b A/a;B/b A/a;b/b
a;B
A/a;B/B A/a;B/b a/a;B/B a/a;B/b
a;b
A/a;B/b A/a;b/b a/a;B/b a/a;b/b
9
:7
98
4.Supplementary Gene Action
In this gene interaction, the dominant
allele of one gene produces a phenotypic
effect.
The dominant allele of the other gene
does not produce any phenotypic effect
on its own;
But when it is present with the dominant
allele of the first gene, it modifies the
phenotypic effect produced by the first
gene.
99
Parents:
RRPrPr
(Purple)
Gametes:
RPr
F1
X
rrprpr
(White)
rpr
RrPrPr
(Purple)
100
Supplementary gene action
In maize, the development of colour is governed
by two completely dominant genes R and Pr.
The dominant allele R is essential for (red)
colour production
Homozygous state of the recessive allele r (rr)
prevent the production of red colour.
Phenotypic ratio: 9:3:4
(Purple: Red: White)
101
RPr
Rpr
rPr
rpr
RPr
• Modified ratio
• Still 1/16ths
Rpr
rPr
9:Purple
3:Red
4: White
rpr
9
:4
:3
102
5. Inhibitory gene action
One dominant inhibitory gene prevents the
expression of other dominant gene.
E.g. ,
There are two varieties of white comb which
are genetically different.
One variety is called Leghorn represented
by IICC, which carries a colour gene C, and
other gene I, that inhibits the expression
of the colour gene C
The other white var. called Wyandotte is
recessive to the two genes viz., iicc and so
it is white or colourless
103
When cross made btn 2 white var. Leghorn
and Wyandotte, F1 hybrid also white, When
this F1 allowed to self fertilize there was a
segregation of 13:3 ratio of white and
coloured respectively.
Parents:
IICC
X
White (Leghorn)
Gametes:
IC
F1
iicc
White (Wyandotte)
ic
IiCc
(White)
104
IC
Ic
iC
ic
IC
Ic
iC
ic
13
:3
105
6.Masking gene action
In barely, seed colour is governed by two
dominant genes B and Y.
The allele B produces black colour, while
Allele b produces white colour
The dominant allele Y produces Yellow
While, allele y produces white colour
106
Parents:
BByy
(Black)
Gametes:
By
F1
X
bbYY
(Yellow)
bY
BbYy
(Black)
107
A;B
A;b
a;B
A;B A/A;B/B A/A;B/b A/a;B/B
a;b
A/a;B/b
A;b
A/A;B/b A/A;b/b A/a;B/b A/a;b/b
a;B
A/a;B/B A/a;B/b a/a;B/B a/a;B/b
a;b
A/a;B/b A/a;b/b a/a;B/b a/a;b/b
12
:3
:1
108
7. Polymeric Gene Action
In Polymeric gene action, two completely
dominant genes controlling a character
produce the same phenotype when their
dominant alleles are alone.
But when dominant alleles of both the
genes are present together, their
phenotypic effect is enhanced as if the
effect of the two genes were cumulative
or additive.
109
Polymeric Gene Action
In barley, two completely dominant genes A
and B affect the length of awns (the needle
like extensions of lemma).
Dominant allele of the gene A or B alone (e.g.,
Aa bb, AA bb, BB aa, Bb aa) give rise to
awns of medium length .
Thus the phenotype of A is same as that of B.
But when A and B together they produce long
awn.
Where as aa bb are awnless.
110
Parents:
AABB
(Long Awn)
Gametes:
AB
F1
X
aabb
(Awnless)
ab
AaBb
(Long Awn)
111
AB
Ab
aB
ab
AB
Ab
aB
9: Long Awn
6: Medium
Awn
1: Awnless
ab
112
8. Additive Gene action
In additive gene action, each positive allele
of the two genes governing a trait
produces equal and identical effect on the
character.
Therefore the genes showing additive gene
action are called multiple factors or more
commonly Polygenes
113
Each polygene has two alleles: one allele of
each polygene produces a +ve effect in the
character governed by the gene. – Positive
allele.
The other allele of each gene has no effect
on the character and is negative allele
114
Seed Colour in tetraploid wheat is governed
by two polygenes R1 and R2, and
determining the colour, where as r1 and r2
do not produce any colour.
Each R1 and R2, allele produces a small
amount of colour.
Therefore the total intensity of colour
depends on the total number of positive
alleles of the two genes present.
Thus,R1R1R2R2 produces dark red colour;
R1R1R2r2 generate medium dark red and
R1r1R2r2 yield medium red and so on.
115
Parents:
R1R1R2R2
(Red)
X
r1r1r2r2
(White)
Gametes:
R1R2
F1
r1r2
R1r1R2r2
(Medium red)
116
1: Dark red
4: Medium dark red
6: Medium red
4: Light red
1: White
117
Summary of Epistasis
118
Multiple Allele
It is generally accepted that gene has two
alternative forms called Allele.
Usually, one of the two alleles of a gene
is dominant over the other, which is
recessive e.g, Tall (TT)
But in many cases, several alleles of a
single gene governing the concerned trait
and is known as Multiple alleles.
119
Both prokaryotes and Eukaryotes show multiple
alleles.
Examples:
1. Human ABO
2. Rh and other blood group
3.Fur colour in rabbits and other mammals.
4. Drosophila eye colour
120
What is blood made up of?
An adult human has about 4–6 liters of
blood circulating in the body. Among other
things, blood transports oxygen to various
parts of the body.
The red blood cells contain hemoglobin, a
protein that binds oxygen. Red blood cells
transport oxygen to, and remove carbon
dioxide from, the body tissues.
The white blood cells fight infection.
The platelets help the blood to clot, if
you get a wound for example.
The plasma contains salts and various
kinds of proteins.
121
What are the different blood groups?
The differences in human blood are due to the
presence or absence of certain protein molecules
called antigens and antibodies.
The antigens are located on the surface of the
red blood cells and the antibodies are in the
blood plasma.
Individuals have different types and combinations
of these molecules.
The blood group you belong to depends on what
you have inherited from your parents.
122
There are more than 20 genetically determined
blood group systems known today, but the AB0
and Rh systems are the most important ones
used for blood transfusions.
Not all blood groups are compatible with each
other. Mixing incompatible blood groups leads
to blood clumping or agglutination, which is
dangerous for individuals.
Nobel Laureate Karl Landsteiner was involved in
the discovery of both the AB0 and Rh blood
groups.
123
Blood group A
If you belong to the blood group
A, you have A antigens on the
surface of your red blood cells
and B antibodies in your blood
plasma
Blood group B
If you belong to the blood group
B, you have B antigens on the
surface of your red blood cells
and A antibodies in your blood
plasma
124
Blood group AB
If you belong to the blood group
AB, you have both A and B antigens
on the surface of your red blood
cells and no A or B antibodies at all
in your blood plasma.
Blood group 0
If you belong to the blood group 0
(null), you have neither A or B
antigens on the surface of your red
blood cells but you have both A and
B antibodies in your blood plasma.
125
Multiple Allele
126
Rh factor blood grouping system
Many people also have a so called Rh factor on the
red blood cell's surface.
This is also an antigen and those who have it are
called Rh+.
Those who haven't are called Rh-.
A person with Rh- blood does not have Rh
antibodies naturally in the blood plasma (as one
can have A or B antibodies, for instance).
But a person with Rh- blood can develop Rh
antibodies in the blood plasma if he or she
receives blood from a person with Rh+ blood,
whose Rh antigens can trigger the production
of Rh antibodies.
A person with Rh+ blood can receive blood from a
person with Rh- blood without any problems.
127
Blood is frequently exchanged between mother and the
fetus during childbirth. Thus, Rh-negative mothers
often immunized by blood from Rh-positive fetuses
(which may result when father is Rh-positive).
Usually no ill effects are associated with exposure of
the mother to the Rh-positive antigen during the
first child birth.
Subsequent Rh-positive children carried by the same
mother, however, exposed to the antibodies
produced by the mother against the Rh antigen.
Such children may develop symptoms of hemolytic
jaundice or anemia, a condition referred to as
erythroblastosis fetalis. .
The symptom may be mild or severe, even resulting in
the death of the fetus.
128
The number of different genotype is possible to
find out in multiple alleles
If n is the number of alleles of a gene, the
number of different genotypes possible is
n(n+1)/2, thus 2,3,4 or 5 alleles there are
3,6,10 and 15 possible genotype respectively.
Note that although a large number of different
alleles of a given gene may be present in a
population or sp., only two of those alleles can
be present in any diploid organism.
129
Alleles
1
2
3
4
5
6
n
Homozygous
1
2
3
4
5
6
n
Heterozygous
0
1
3
6
10
15
n(n-1)/2
Genotype
1
3
6
10
15
21
n(n+1)/2
130
Coat colour in Rabbit
Multiple alleles involves coat colour in
rabbits.
Four alleles of rabbit coat colour (c) gene
have been studied:
C+ wild type or full colour
Cb “himalayan” white with black tips on the
extremities
Cch “chinchilla” mixed colour & white hairs
C albino
These alleles show a gradation in
dominance of C+ > Cb > Cch >C
131
Multiple allele in Drosophila eye colour
Wild type Drosophila have red eyes;
But a vast array of eye-colour mutants have
been studied extensively.
Mutant alleles of one gene (white) result in flies
with eye colour ranging from pure white
through a series of intermediate colours up to
nearly wild type red when present in
homozygous condition.
For e.g., the allele that produces the most
extreme mutant phenotype was named white
because the eyes of the flies homozygous for
this allele are completely white (no coloration)
132
Other alleles of the gene were designated wa
(white apricot)
we (white eosin), wch (white cherry), wco (white
coral), ww (white wine), wb (white blood), wcrr
(white carrot), wcf (white coffee) and so on,
to indicate that different levels of coloration
of the eyes occurred in flies homozygous for
each particular allele.
133
Codominance Problem
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IB
IB
IA
IAIB
IAIB
i
IBi
IBi
1/2 = IAIB
1/2 = IBi
134
Another Codominance Problem
Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
135
Try These
1. If a male has blood type B and a
female has blood type A, what are the
possible blood types in the offspring?
2. Is it possible for a child with Type O
blood to be born to a mother who is
type AB? Why or why not?
3. A child is type AB. His biological
mother is also type AB. What are the
possible phenotypes of his biological
father?
136
Linkage
Discovery of Linkage:
In 1900, Mendel’s work was re-discovered, and
scientists were testing his theories with as
many different genes and organisms as
possible.
William Bateson and R.C. Punnett were working
with several traits in sweet peas, notably a
gene for purple (P) vs. red (p) flowers, and a
gene for long pollen grains (L) vs. round pollen
grains (l).
137
observe deviations from
9 : 3 : 3 : 1 ratios derived from
dihybrid crosses
1 : 1 : 1 : 1 ratios derived from
test crosses
too many
too few
138
It is universally accepted that genes are
located in chromosomes
During cell divisions each chromosome
behaves as a unit
Therefore,
genes
located
in
same
chromosome would move together to the
same pool during cell division.
Linkage: The tendency of two or more
genes
to
stay
together
during
inheritance is known as Linkage.
Because they are located on same
chromosome.
139
Recombination During Meiosis
Recombinant gametes
Parental gametes
140
Linkage in Drosophila
In Drosophila Grey
colour is dominant (G
or b+) over black
colour (g or b) and
long wing (L or Vg+)
over vestigial wing (l
or Vg).
The genes of these
characters are linked
together in same
chromosome.
ebony body, vestigial
wild-type female winged male
141
142
143
Coupling and Repulsion
In Maize,
Dominant gene C produces coloured seeds (C)
Recessive gene produces colourless seeds (c)
Another dominant gene governs full seeds (Sh)
While recessive produce shrunken seeds (sh)
Parents
CCSh
Gamets
C Sh
F1
X
ccsh
c sh
CcShsh
144
Test cross:
When F1 are test crossed with recessive
strain, out of 8,368 seeds 4,032 (48.2%) were
coloured full; 4,035 (48.3%) were colourless
shrunken; 149 (1.7%) were coloured shrunken;
152 (1.8 %) were colourless full.
Here, the four phenotypic classes are not in the
expected ratio of 1:1:1:1.
The phenotypic classes coloured full and colourless
shrunken have a much higher frequency than the
expected 25%.
These two character combinations are called
Parental phenotype or parental types or Parental
combinations.
145
In the above e.g., it appears as if the 2 dominant
genes C and Sh have a strong affinity for each
other so that the phenotypes are greater than
expected.
This situation is referred to as Coupling phase.
It is due to the presence of genes C and Sh in the
same chromosome.
146
Similarly when coloured shrunken seeds (CC shsh)
were crossed with colourless full (cc ShSh), F1
were coloured full (CcShsh).
But when F1 test crossed with recessive strain (cc
shsh) 47.9% were coloured Shrunken; 49.1%
colourless full; 1.4% coloured full; 1.5% colourless
shrunken
In this case also the parental types were more
frequent.
It appears as if in this cross the dominant genes c
and Sh dislike each other.
This situation is referred to as Repulsion phase.
It is due to the presence of dominant allele of one
gene, e.g., C with the recessive allele of the
other gene, e,g, sh in the same chromosome.
147
Complete and Incomplete linkage
Gene show a linkage because they are located on a
chromosome.
For e.g., C and Sh are present in one chromosome, while
their recessive alleles c and sh are situated in the
homologous chromosome.
C
sh
c
Sh
C
sh
c
Sh
148
Each chromosome behaves as a unit during cell
division.
Therefore, C and Sh would move to one pole to
while c and sh move to opposite pool.
If this always happened, F1 generation (CcShsh)
would produce 2 gametes viz., C Sh and c sh
When only parental character combinations are
recovered in test progeny , it is called
complete linkage.
However, sometime, allele recombine to produce
recombinant types like C sh, cSh
Such a type are called Incomplete linkage.
149
Linkage is never 100%. No matter how
tightly two genes are linked, if you
observe enough individuals, you will find
some recombinants.
150
Crossing Over
Recombinant phenotypes are produced by
recombinant gametes. These gametes, in turn,
are produced due to crossing over.
The exchange of homologous segments between
nonsister chromatids of homologous
chromosomes is known as Crossing over.
Crossing over takes place during pachytene .
151
Centromere
sister
chromatids
sister
chromatids
chiasmata
{
{
152
Crossing over occurs during meiosis of gametogenesis.
The homologous chromosomes move towards each other and
come to lie side by side.
This phenomenon of pairing of homologous chromosomes is
called synapsis.
The paired homologous chromosomes are called bivalents.
The homologous chromosome split into two chromotids
153
154
Chiasmata mark the sites of recombination
155
Chiasmata
mark the sites
of
recombination
156
chiasmata: visible
manifestations of crossing
over
Proposed by Janssens
Prior to crossing over, the
chromosomes of each
bivalent get duplicated to
form a tetrad.
Crossing over occurs only
between the nonsister
chromatids of a tetrad.
In this stage, the nonsister
chromatid overlap with one
another and form chiasma
or point of contact
157
Recombination Frequency
RF =
recombinant progeny
X 100%
total progeny
158
Salient features of Crossing over
1. Crossing over is the interchange of chromosomal
segments between the homologous chromosome
2. It occurs during meiosis or gametogenesis
3. The crossing over occurs only between nonsister
chromatids of the homologous chromosome
4. The number of crossing over is depends upon the
length of the chromosome. The longer the length,
higher percentage of crossing over
5. When the genes are located apart from one
another the chances of crossing over is higher
when the genes are closely located the chances
for crossing over is lesser.
159
RECOMBINATION
products of meiotic recombination
160
Detection of recombination using a test cross
161
Kinds of crossing over:
1. Single crossing over
In this type, only one chiasma is formed.
Only one chromatid of each chromosome is
involved in single crossing over.
2. Double crossing over
Here two chiasma are formed. The
chiasmata may be formed between the same
chromatids or between different chromatids.
Thus 2 or 3 or all the four chromatid may be
involved.
3.Multipel crossing over:
More than two chiasma are formed
162
Crossing over occurs in the Tetrad Stage
Neurospora crassa highly suited certain types
of genetic studies.
1. After two haploid nuclei from cells of two
different matting types fuse to form a diploid
nucleus (similar like fertilization in higher
org).
2. The haploid products of meiosis, called
ascospores, are maintained in linear order
within an elongate tube like structure called
ascus (plural asci).
Thus each ascus contains all four products of
a single meiotic event.
Moreover all of the ascospore in each ascus
can be analyzed genetically.
163
3. These haploid ascospores germinate and grow
to produce multicellular mycelia (Singular
mycelium) and all of these cells are also
haploid.
The genotype of each product can thus be
determined without carrying out the test
cross or other genetic manipulations.
Because of the haploid state of the mycelium,
the presence of recessive marker is never
masked by dominant alleles.
4. Neurospora can be grown on a simple
synthetic media
164
Neurospora crassa life cycle
165
Cytological basis of Crossing over
Normally, the two chromosomes of any homologous
pair are morphologically indistinguishable.
Stern, Creighton and B. McClintock, however
identified homologous that were morphologically
distinguishable, that is they were not entirely
homologous,
Stern(1931)obtained a type of female Drosophila in
which two X chromosomes are different from each
other and also from other sets of chromosomes.
He found that X chromosome in which a piece of Y
chromosome is attached.
The second X chromosome has been broken into two
unequal segments and is shorten than the unbroken
X chromosome
166
Both the chromosomes can be distinguished by
microscopic examination.
The broken C chromosomes contain a recessive
gene (c) for carnation eye colour and the other
chromosome contains dominant allele (C) red
eyes.
In addition, the broken X chromosome contains a
dominant gene (B) for narrow bar eyes while,
homologous contain recessive allele (b) for
normal round eyes.
Stern crossed this female having red bar eyes with
a double recessive male having carnation round
eyes.
167
He selected only females for his experiment.
In the absence of crossing over, only two types
of female gametes are produced:
One type having broken X containing
c and B genes;
Other type having X with piece of
Y attached containing C and b.
But if crossing over occurred two
More types of gametes are produced:
One type having c and b
in a normal size X
and the other type having
C and B on a
broken X and with a piece of Y
Chromosome.
168
Stern’s: crossing over in Drosophila
F1 female
car+
B+
car
B
Abnormality at
another locus of
X chromosome
car
B
No
crossing
over
car
B
car+
B+
Abnormality at
one locus of
X chromosome
car
B
car+
Crossing over
B+
during meiosis
in F1 female
car
B+
car+
B
car
B+
Fertilization
by sperm
from carnation
F1 male
car
B+
car
B+
car car
B B+
car+car
B+ B+
carnation, normal
bar
car+
B
car car
B+ B+
carnation
Fertilization
by sperm
from carnation
F1 male
bar
Parental combinations of
Recombinant combinations
both genetic traits and
of both genetic traits and
chromosome abnormalities chromosome abnormalities
169
Factors affecting crossing over
1. High temperature increases the frequency of
crossing over
2. X-ray increases the frequency.
3. The frequency crossing over decreases with
increasing age in female Drosophila
4. Crossing over is less frequent near centromeres
and the tips of the chromosomes.
5. Nutritional and chemical effect:
170
Linkage Mapping
Each gene is found at a fixed position on a
particular chromosome. Making a map of their
locations allows us to identify and study them
better.
In modern times, we can use the locations to
clone the genes so we can better understand
what they do and why they cause genetic
diseases when mutated.
Thus, the percentage of gametes that had a
crossover between two genes is a measure of
how far apart those two genes are.
171
Alfred Sturtevant, a student of Morgan’s was
assigned a project of consolidating the early
information on D. melonogaster.
He conceived an approach in which recombination
data would be used to describe the physical
relationship of genes on a chromosome in a linear
arrangement called linkage map or genetic map.
172
For example, here 11.26% of the offspring
from the cross in Table were recombinant
between genes pp and ll, making the map
distance between them 11.26 map unit.
173
As pointed out by T. H. Morgan and Alfred
Sturtevant, who produced the first Drosophila
gene map in 1913. Morgan was the founder of
Drosophila genetics, and in his honor a
recombination map unit is called a centiMorgan
(cM).
A map unit, or centiMorgan, is equal to crossing
over between 2 genes in 1% of the gametes.
174
Morgan and his students discovered a number of genes in
D.melonogaster and examined their progeny, they found
substantial variation in the frequency of recombination for
different gene pair.
Genes
Yellow (Y) – White (W)
Yellow (Y) – Vermilion (V)
White (W) – Vermilion (V)
Vermilion (V)- Miniature(M)
White (W) –Miniature(M)
White (W) – rudimentary (r)
Vermilion (V)– rudimentary (r)
Recombination
frequency
214/21.736
1,464/4551
471/1,584
17/573
2,062/6116
406/898
109/405
= 0.010
= 0.322
= 0.297
= 0.030
= 0.337
= 0.452
= 0.269
175
Sturtevant constructed a map beginning with Y at 0.0 on left
side and using the frequencies of recombination between
adjacent genes.
Y W
* *
0.0 1.0
V
M
*
30.7
*
33.7
r
*
57.6
176
Three Point Crossing over
177
Three – Point Crossing over
Assuming that 3 genes
involved are arranged in a
linear fashion; there are 3
possible order.
Each having a different gene
in the center.
With 3 linked genes, there are
3 different recombination
frequencies between pair of
loci: between v and ct;
between ct and cv; between v
and cv.
178
Genotype
v cv+ ct+
v+ cv ct
v cv ct+
v+ cv+ ct
v cv ct
v+ cv+ ct+
v cv+ ct
v+ cv ct+
total
#
580
calculate distances:
592
2 genes at a time;
45
e.g.: v  ct =
40 [ ( 89 +3) / 1448 ] x 100
89
94
3
5
1448
179
Drosophila ...
180
# gene isolation ...
# mapping 1st step…
# tomato…
chromosome
numbering
map (ca. 1952)
181
Interference
RF is defined as the proportion of exchange
between two linked genes.
As we seen more than one recombinational exchange
may occur on a chromosome.
A standard way of describing the difference
between the observed and expected numbers of
double crossing over was first used by H.J.Muller.
Interference is
I=
Observed frequency of double recombinants
1Expected frequency of double recombinants
182
Positive value indicated interference between
recombination recombination events.
While, values near zero suggesting no
interference, that is that different
recombinant events are independent to each
other.
For closely linked genes, the measure of
interference may often be near one, while for
widely separated genes on the same
chromosome, I is often near zero.
183
Try this
184
Mapping: Locating genes along a
chromosome
185
SEX DETERMINATION
What determines Maleness and femaleness?
Nature encompasses a vast array of diverse mechanisms
of sex determination in different species.
Generally in any organism, individual normally exhibit one of
the two sex phenotype: female or male.
In such sp, females produce female gametes (egg, ovules or
macrospores) and males produce male gametes (sperm,
pollen or microspores)
Dioecious : Sp with separation of the sexes in different
individuals are called dioecious – all the higher animals
and some higher plants are e.g.
Monoecious: Sp in which both male and female gametes are
produced by each individual are monoecious,
186
In lower animals, the production of both eggs and
sperms by the same organisms is more commonly
called hermaphroditism, and individual organism
producing both types of gametes are termed
hermaphrodites.
Although the two sex phenotypes are usually quite
distinguishable in human and fruit flies, this is
not universally the case.
In lower eukaryotes, the two genetically distinct
types of gametes are sometimes morphologically
indistinguishable; this is called isogamy. E.g.,
green alga Chlamydomonas
187
Sex Chromosomes
At the cellular level the sex of an individual is
determined genetically by the sex chromosomes.
XX = female, XY = male
All other chromosomes are called
“autosomes”
Thus, humans have
46 chromosomes, (44+2)
188
Human males are the
heterogametic sex with two
different sex chromosomes,
(XY).
Human females are the
homogametic sex (XX).
In other species sex
determination differs:
male birds
ZZ
female birds
ZW
189
What is so different between the X and Y
chromosomes?
X- over 1000 genes identified
Y- 330 genes identified, many are inactive
One gene on the Y is very important: SRY. The SRY
gene is the primary determinant of sex.
If SRY is present, testes develop in the early embryo.
The testes secrete the hormone testosterone, which
causes development as a male.
If SRY is absent (no Y chromosome), ovaries develop
instead of testes, and the embryo develops into a
female.
190
Genes on the Y chromosome
There are three classes of
genes on the Y.
Genes shared with X
chromosome define the
pseudoautosomal regions
(PAR)
Genes similar to X chromosome
genes are X-Y homologs
Genes unique to the Y including
SRY gene
191
What is SRY?
SRY – Sex determining Region of Y
TDF – testis determining factor
SRY – starts male development by
- turning on testis-determining genes
- turning off ovary-determining genes
192
Embryology of internal reproductive organs
193
1.Initiates the process
that directs the indifferent
gonads toward testis
development
2. Activates Sertoli cells to
produce Mullerian
inhibiting hormone,
causing Mullerian duct
degeneration
194
Stimulates Leydig cells to
secrete testosterone, which
then directs
development of the Wolffian
ducts towards epidiymides,
vas deferens and seminal
vesicles
- Testosterone conversion to
dihydrotestosterone (DHT)
- directs development
of the urethra, prostate
gland and Male sex organ.
195
Y chromosome ( SRY region;TDF gene) is not
present.
- no TDF to tell it to form testis
- gonadal tissue develops towards ovary formation
- In the absence of testosterone – Wolffian duct
system degenerates
- In absence of MIH – Mullerian ducts continue to
develop towards fallopian tubes, uterus, and
upper vagina.
196
The different mechanisms of chromosomal
sex determination may be grouped into
five classes:
1. XX female, XO male
2. XO female , XX male
3. XX female, XY male
4. XX male, XY female
5. Diploid (2n) female, haploid (n) male.
197
1. XX female, XO male
In grasshoppers, protenor and many other insects
belonging to Orthoptera, females have two X
chromosomes, while males have only one X.
Consequently, somatic cells of female have one
chromosomes more than those of males.
Here females are homogametic sex i.e., the sex
producing only one type of gametes with respect
to the sex chromosomes.
In males, the single X chromosome remains
unpaired and half of the sperms produced by
males have one chromosome, while the other have
none.
198
2. XO female; XX male
This system of sex determination is known in some
insects species only, e.g., Fumea.
In such sp., females have only one X chromosome.
As a result they produce two types of eggs, half
having an X chromosome and other half having
none.
The males, on the other hand, have two X
chromosomes; all the sperms, therefore have one
X chromosomes.
Here male – homogametic sex
Female – heterogametic sex.
199
3. XX- female; XY- male
This system is most common among animals.
It is found in humans, mice, most other mammals,
etc.
Male- XY
Female – XX
Y chromosome differs in morphology from the X
chromosome, and generally smaller than X.
200
4. XY- female, XX- Male
This system of sex determination operates in
birds, reptiles, some insects (Silk warm).
The scheme is essentially opposite to mammals.
The bird sex chromosomes are called Z and W.
In birds, a ZZ individual is male, and a ZW
individual is female.
201
Diploid (2n) female, Haploid (n) male
This system of sex determination is found mainly in
Hymenoptera – honey bees, ants, termites, etc.
In these sp, the somatic chromosome number of
female is diploid, while that of males is only
haploid.
The unfertilized haploids develop male and called
drones.
These drones carry only half the number (16) of
chromosomes of the female (32)
202
Environmental:
Egg size:
In the case of sea worm, Dinophilus, the size of
eggs appears to determine the sex of animals
developing them.
Animal developed from eggs of relatively larger
size are females, while those obtained from
comparatively smaller sizes are males.
203
Association with females:
The larvae of the sea worm Bonelia are sexually
undifferentiated.
These larvae that attach to the proboscis of
female worms develops into males.
In contrast, those larvae that do not attach to
female remains free living female
204
Temperature:
Some species have environmentally determined
sex. Among reptiles, the temperature at which
the eggs develop determines the sex.
For example, in the turtles, eggs incubated at 30oC
become female, while those incubated at lower
temperatures become male.
205
Sex index or Genic balance theory
Formulated by Bridges.
According to this theory, sex is determined by the relative
number of X chromosomes and autosomes.
Its actually the ratio between X chromosome and
autosomes.
Thus,
Number of X chromosome
Sex Index =
Number of haploid sets of autosomes
If the sex index is 1 = female
0.5 = male
intermediate = intersex
Above 1 = meta female
less than 0.5 = meta male
206
Gynandromorphs
In Drosophila, some individuals show male
characteristics in a part of their body, while
their remaining parts show the female
phenotype.
Such a individuals are known as Gynandromorphs.
Gynandromorphs are believed to arise from XX
zygotes .
During embryonic development, in one or more cells
one of the two X chromosomes does not pass to
any pole at anaphase and as a result, is lost.
207
Sex determination in Plants
The mechanism of sex determination in plants are similar
to animals
Environmental sex determination:
In some plants, either sex determination is due to envt.
Or it is greatly affected by envt.
Equisetum plants grown under optimum conditions became
female and while those grown under adverse conditions
became male.
In many other plants like, melons, cucumber, Cannabis etc,
the sex of the flower is affected by many
environmental factors such as temperature, day length
and ethylene, GA3, etc.
In general, a treatment with GA3 or ethylene promotes
production of female flowers.(e.g, Cannabis)
208
Chromosomal sex determination in plants
Many dioecious plants do not show distinct sex
chromosomes. But some of the sp, do show clear
cut chromosome.
(i) XX female, XY male: Asparagus, Spinach
(ii) XY female, XX male: Fragaria elateria
In Melandrium the Y chromosome determines
maleness , while X chromosome specifies
femaleness.
A single Y chromosome is able to produce male
flowers even in the presence of four X
chromosomes (XXXXY), but such plants produce
some bisexual flowers.
209
The Y chromosome of Melandrium is the largest
chromosome of its genome.
It has four functionally distinct segments.
The first segment contains female suppressor and is
located at one end of the Y chromosome.
A deletion for this segment produce hermaphrodite
flowers on XY chromosome.
The second region, lying next to first, has the genes
that promote maleness, i.e., initiate the development
of the anthers.
The third segment contains genes governing male
fertility and anther maturation.
A deletion of this segment produce male sterile plants
with abortive anthers.
The last segment located end of the Y chromosome, is
homologous to a segment of X chromosome.
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Sex Linked Inheritance
Chromosomes are the carriers of genes. Genes are
located on both the autosomes and sex
chromosomes.
Genes on
Autosomes – somatic characters
Sex chromsomes- sex characters
However, certain genes present in the sex
chromosomes control the somatic characters.
The characters which are controlled by such genes
are called sex linked character and transmission
of such characters from one generation to next
is called sex linked inheritance
211
Discovery and types of Sex linked inheritance
Was first discovered by Thomas H. Morgan in
1910 on Drosophila melanogaster.
It can be classified into three types depending
upon the chromosomes (X or Y) having sex linked
genes.
1. X – Linked inheritance
2. Y - Linked inheritance
3. XY - Linked inheritance
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1. X- Linked inheritance
Certain sex linked genes are located only on X
chromosomes and their alleles are absent in Y
chromosomes.
These genes are called X – linked genes;
Their mode of inheritance is called X-linked
inheritance
This pertains to the inheritance of those
characters which are controlled by genes located
in the non-homologous part of X-chromosomes
Examples: Colour blindness, Hemophilia – human
Eye colour in Drosophila
213
Y – Linked inheritance
The sex linked genes are located on Y chromosomes
only are called Y linked genes.
The Y linked genes are confined only male (human).
Hence they are also called Holandric genes (holo –
whole; andros –male)
These genes are transmitted directly from the
father to the son
Example: Hairy ear rims in Men
214
XY Linked inheritance
Certain sex linked genes are located on both X and
Y chromosomes
They are called XY linked genes and their mode of
inheritance is called XY linked inheritance.
215
General patterns of Sex linked Inheritance
X – Linked Recessive:
1. Usually more males than females are affected
2. No offspring of an affected male are affected,
making the trait skip generations in pedigree.
But an exception to this pattern occurs in the
rare instance when the affected male mate with
a female carrier, producing a affected female
offspring.
X – Linked Dominant:
1. Affected male produce all affected female
offspring and no affected male offspring
2. Approximately half the offspring of affected
females are affected, regardless of their sex.
216
Y – Linked Inheritance:
1. Traits are always passed from father to son
2. Only males are affected.
Kinds of Sex Linkage:
1. Diagenic: when the characters pass from father
to grandson through his daughter
Father → Daughter → Grandson
2. Diandric: When the characters pass from
mother to grand-daughter through her son
Mother → Son→ Granddaughter
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3. Hologenic:
when characters go directly from female to
female, i.e., from mother to daughter and then to
granddaughter.
Mother → Daughter → Granddaughter
4. Holandric:
When the characters are pass from male to
male, i.e, from father to son and then to his
grandson.
Father → Son → Grandson
218
Sex linkage in Drosophila:
Normal wild type of fruit fly has
red eye (RR).
In the population of red eyed flies,
white eyed (recessive) appeared as
mutation (rr).
When white eyed male was mated
with red eyed female F1 flies had
red eyes
In F2 on an average of 3 flies had
red eyes and 1 had white eyes
219
This results reveal white eye is due to recessive
gene.
But if F2 individuals are classified on the basis of
their sex as well as eye colour, a peculiar picture
is emerged.
All the F2 female had red eyes, but half of the
males had red eye and the other half had white
eyes.
It appears that eye colour of a fly in F2 depended
on its sex
220
But when white eyed female
flies were mated with red
eyed males, half the flies
were red-eyed and the
remaining half had white
eyed.
In F2 generation, both male
and female flies showed the
ratio of 1 red: 1 white.
Morgan correctly reasoned
that white eye gene was
located on X chromosome
221
Colour blindness
We
have
3
color
receptors
in
the
retinas of our eyes.
They respond best to
red, green, and blue
light.
Each
receptor
is
controlled by a gene.
The blue receptor is on
an autosome, while the
red
and
green
receptors are on the X
chromosome
(sexlinked).
222
Features of Colour blindness
Colour blindness is a sex linked character discovered by
Wilson
It is a hereditary disease and the affected person cannot
distinguish green and red colour.
The red blindness is called Protonopia, these persons cannot
see red colour
While, green blindness is called deuteronopia, such a persons
cannot see green colour.
Colour blindness is a recessive character, represented by cc
The genes are for colour blindness is located on X
chromosome. It is common in male but rare female.
Colour blindness follows criss- cross inheritance as
transmitted from father to grandson through daughter.
It is never transmitted from father to son
223
XCXC
- Normal female
XCXc
-
XCY
- Normal Male
XcY
- Affected male
XcXc
- Affected female
Carrier female
224
225
Hemophilia
Hemophilia is another sex-linked trait.
Discovered by John Cotto.
Hemophilia is characterized by delayed blood
clotting. People with hemophilia can easily bleed
to death from very minor wounds.
Otherwise called as “Bleeders disease”
Hemophilia is sex linked recessive character (hh)
and the genes are located on X chromosomes
It is common in men and rare in women
226
This disease is
appeared as a mutant
in Queen Victoria and
from her it was
transmitted to her
descendants.
“Royal disease”
XH XH
XH Xh
XH Y
Xh Y
Xh Xh
- Normal female
- Carrier female
- Normal Male
- Affected male
- Affected female
227
Sex linked in Cat
Sex linkage has been noticed in
Tortoise shell cats.
This involves the inheritance of coat
colour.
Tortoiseshell cats have patches of
black and orange fur. Almost all
tortoiseshells
are
female.
Heterozygous for the X-linked ♀
♂
coat color gene, one allele black
and the other allele orange.
XB XB X Xb Y
When a black coloured female is
(Yellow)
matted with yellow coloured male, (Black)
tortoise shell female is obtained.
XBXb
Even reciprocal also tortoise are
(Tortoise shell)
female
228
Incomplete Sex Linkage
The genes located on homologous regions of sex chromosomes
do not inherit together because crossing over occur may
occur in these region.
So these genes are incompletely sex linked genes and their
mode of inheritance are incomplete sex inheritance.
Xeroderma pigmentosum is an abnormal condition of skin in
man, in which the skin becomes extremely sensitive to light.
Even low light can produce pigmentation. In extreme cases,
develops cancerous growth in body.
Retinitis pigmentosa is another case of incomplete sex linkage
caused by the interaction of a number of dominant and
recessive genes.
In this case, the affected person develops night blindness
leading to the formation of pigmentation in retinal wall.
229
Sex Limited Traits
Some genes fail to produce their phenotypic
effects in the presence of certain hormones and
are described as sex limited.
In human being several characters are restricted
with particular sex.
For e.g, facial hair in male only (beard )
230
Sex-Influenced Traits
In contrast to Sex limited gene where
one expression of a trait is limited
to only one sex only; sex influenced
genes are those whose dominance is
influenced by the sex of the bearer.
Such characters are called Sex
influenced traits.
Good examples: male pattern baldness
in humans and horns in sheep.
Pattern baldness is found in both
sexes, but is rarer in females, but it
is dominant in males and recessive in
females. Thus, male heterozygotes
are bald but female heterozygotes
have normal hair.
231
The Adams family and Baldness
232
Dominant sex influenced trait
Autosomal genes responsible for horns in some breeds of
sheep may behave differently in the presence of male and
female sex hormones.
In suffolk sheep neither sex is horned, and the genotype is
“hh”.
Among the heterozygous F1 progeny crosses between these
breeds, horned males and hornless female are formed.
Because both the sex are genotypically alike (h+h), the genes
behave as dominance in male and recessive in females, i.e.,
only one allele is required for male but the allele must be
homozygous expression in female.
Genotype Males
Females
h +h +
h +h
hh
Horned
Horned
Horned
Hornless
Hornless Hornless
233
Deleterious recessive Sex Linked Genes in Human
1. Lesch –Nyhan Syndrome:
Characterized by excess of uric acid, is inherited
through sex linked recessive gene.
This means that mother contributes the X chromosome with
defective gene to a male zygote.
Half of the male children of carrier mothers may be
expected to inherit this disease.
These are deficient for the enzyme hypoxanthine-guanine
phosphoribiosyltransferase.(HPRT)
Infant who receive this disease appears normal at birth and
for several months.
By about 10 months of age, they become abnormal and
mental retardation, and patient may literally bite of
his own fingers and lips.
234
2. Duchene type Muscular Dystrophy:
Is also depends on sex linked recessive gene.
If mother is carrier, about half of the male
children are expected to be affected.
Can be identified by chromosome study.
It affects male before they reach teens, with
muscular deterioration .
Muscles of leg and shoulders become stiff and the
children usually become paralyzed and crippled
during their middle or late teens.
Virtually all die before age of 21.
235
3. Hunter syndrome:
It is characterized by mental retardation,
coarse features, hirsutism (abnormal hairiness),
and a characteristic broad nose and a large
protruding tongue.
Symptoms appear in childhood, and a chemical
procedure for diagnosis has been developed.
Mucopolysaccharides (Containing an amino sugars
as well as uronic acid) accumulate in skin cells of
these patients.
Stained with O-toluidine blue, will results in pink
in colour.
236
EXTRA CHROMOSOMAL INHERITANCE
Genes controlling the phenotypes of eukaryotes are
contained in nucleus.
This was certainly the case for the genes studied
by Mendel, and for most, if not all.
“However, the nucleus is not the whole story”
Several important phenotypes of eukaryotes are
controlled by genes in the cytoplasm.
The genes that are present on the cytoplasm are
called Plasmagenes.
Plasmagenes also transmit character from one
generation to next
This is known as cytoplasmic or extrachromosal
inhreitance
237
The first good example of cytoplasmic inheritance
emerged from Carl Corren’s studies of the four
–O’ Clock plant and from Erwin Baur’s work on
Pelargonium in 1909.
Both Mirabilis and Pelargonium frequently have
variegated leaf colour, showing patches of green
and white.
However, certain strains of both the sp can be
either all green or all white (though the white
strains do not survive very long).
238
Non-Mendelian inheritance
In Mendel expt., it made no difference which
parent carried which genotype.
For eg., yellow seed (male) X green (female)
Will give the same result as reverse.
But with leaf colour with Mirabilis it made a great
difference: When Correns crossed any male
(white or green or variegated) with green female
shows all progeny were green.
Similar with white female.
This pattern of inheritance is called Maternal
inheritance
239
This behavior violates Mendel’s law in 2 ways:
1. Usual Mendelian ratio of phenotypes were not
observed
2. There was a difference between reciprocal cross
How do we explain this phenomenon?
240
The situation become much clearer when we realize
the colour in the plant is due to chlorophyll in the
chloroplast.
Genes are governing in the chloroplast and not in
the nuclear gene.
But you can ask why such maternal parent
dependence?
This is because the maternal parent contributes
most of the cytoplasm
241
Kappa particles in Paramecium
Sonnerborn found that there are two strains of
paramecium
They are killer and sensitive
Killer strain produces a toxic substances
“Paramecin” that kills the other type.
The production of paramecin in killer is type is
controlled by certain cytoplasmic particles is
known as “Kappa” particle (KK).
The sensitive strains lack these kappa (kk).
Kappa particles can pass from one generation to
next. They transmitted through the cytoplasm.
242
When killer KK conjugate with non-killer kk, the
ex-conjugants are Kk.
But the development of a particular type depends
on the duration of cytoplasmic exchange.
In a normal case of conjugation the nuclear
material alone is exchanged and as a result each
conjugate gives rise to the organism of its own
type.
i.e., killer exconjugant produce killer while, nonkiller will produce non-killer type.
243
But sometime the conjugation period is prolonged
In such case apart from nuclear material,
cytoplasmic material are also exchanged.
During this cytoplasmic exchange the kappa
particles present in the cytoplasm of the killer
type will enter into the non killer type and
convert it into a killer type.
244
Shell Coiling in Lymnea
The snail Limnaea shows two types of
coiling namely dextral (towards right
side) and sinistral (towards left side).
The dextral coiling is due to dominant
alleles “DD” while the sinistral coiling
due to recessive allele “dd”
The experiments of Boycott indicate
that the coiling is determined by the
gene of the mother and not by the
individuals own gene.
If a dextral female crossed with
sinistral male, all F1 offspring develop
dextral shell like mother
245
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