Exception To Mendelism

This presentation describes about the non mendelian facts in genetics.

1. A Seminar
on
Exception to Mendelism

2. The search for hereditary characters dates back nearly a century; 19
th
century.
1866 Mendel: Introduces concept of heredity
1900
Bateson: Introduced the term Genetics 1906
Wilhelm Johannsen: Coins the term Gene 1909
DeVries, Correns and Tschermak
rediscovered Mendel's work

3. Mendel's Postulates Revisited
Inheritance of biological characters were determined by Genes.
Genes come in pairs (one from each parent).
During sexual reproduction, genes are passed from parents to offspring.
Genes come in different versions, now called alleles. When an organism
has two different alleles of a gene, one (the dominant allele) will hide the
presence of the other (the recessive allele) and determine appearance.

4. During gamete production, each egg or sperm cell receives just one of the
two gene copies present in the organism, and the copy allocated to each
gamete is random (law of segregation).
Genes for different traits are inherited independently of one another
(law of independent assortment).
Cont…

5. Mendel’s Laws Not Perfect


Shortly people began to notice that not all traits are “Mendelian”

This means, they do NOT follow Mendel’s laws

6. Altering Mendel’s Ratios

Two different types of complications:
1. Genotypic ratios follow Mendel’s laws, but phenotypes do not
Somehow the underlying genotypic ratios are hidden

2. Mendel’s laws do not apply
Both genotypes and phenotypes are not following Mendel’s laws

8. In incomplete dominance, neither allele is dominant so there is a blending of traits
when two different alleles for the same trait occur together.

Colors blend together

Heterozygous individuals = 3
rd
phenotype

Phenotypic and Genotypic ratio: 1:2:1
red white pink
Incomplete Dominance or Blending Inheritance
A cross between red and white flowered
plants produced plants with intermediate
flower colour i.e. pink colour in F1 and
a modified ratio of 1 red: 2 pink: 1
White in F2.

9. Fig: Mirabilis jalapa

10. Co-dominance
Both alleles contribute to the phenotype of the organism by showing up simultaneously
(at the same time) in heterozygous individuals.
Neither alleles are dominant.
Heterozygotes expresses both alleles equally.
Phenotypic and Genotypic ratio are equal; 1:2:1
In cattle and horses, if a pure red (RR) is crossed with a pure white(WW), roan (RW) is
obtained.
 These cattle or horses actually have both red and white hairs intermixed, or are spotted.
Roan is a third phenotype.

11. (WW)
(RR)
(RW)

12. Polygenic Inheritance
Some traits are determined by the combined effect of two or more pairs of genes.
These traits are called polygenic traits.
 These traits are not controlled by a single gene locus, but by the combined interaction
of many gene loci.
Characters show a range of continuous phenotypes instead of discrete, defined
phenotypes
In this case many genes have an additive effect. The characteristic or trait is the result
of the combined effect of several genes.
Because so many alleles contribute to the final phenotype, a variety of phenotypes can
occur.

13. Ex: human skin color, eye color, hair color and height.


Note: Traits all result from the interaction of the genes with environmental factors

Fig: Pigmentation in humans is controlled by at least three (3) separately inherited genes

17. Human Eye Color

18. 0
1
2
3
4
5
6
BlackDark BrownLight BrownBlue Light Blue
Proportion of Variants

Skin Pigmentation
×
AaBb AaBb
1
16

4
16

6
16

4
16

1
16

Fig: Histogram representation of skin pigmentation

19. Multiple Alleles
Multiple alleles are gene that are members of the same gene pair and are
located on the same locus.
All of them control same character but each of the allele effects that character
somewhat differently than the others.
Traits are determined by the combined effect of two or more alleles.
Blood type in humans is an example of this inheritance pattern.
 The four different blood groups: A, B, O, and AB
Produced by three different alleles: A, B, and O

20. Bloodtype Alleles
Allele I
A
is dominant
Allele I
B
is dominant
Allele I
O
is recessive
Genotypes Phenotypes
(blood types)
I
A
I
A
A
I
A
I
O
A
I
B
I
B
B
I
B
I
O
B
I
A
I
B
AB
I
O
I
O
O
Note: The phenotypes varies in every
different crosses

21. Examples of Blood type crosses

22. Other examples
Coat color in rabbits
Four phenotypes and four alleles
Allelic series is C > c
ch
> c
h
> c
(which is most dominant)
Phenotype

Genotype
Full Color CC, Cc
ch
, Cc
h
, Cc
Chinchilla c
ch
c
ch
, c
ch
c
h
, c
ch
c
Himalayan c
h
c
h
, c
h
c
Albino cc

23. Lethality
Gene, which causes the death of its carrier when in homozygous condition is called
lethal gene.
The fully dominant lethal alleles kill the carrier individuals.
Mendel’s findings were based on equal survival of all genotypes.
In normal segregation ratio of 3:1 is modified into 2:1 ratio.


E.g: In mice, yellow coat colour is dominant to grey. Mice that have YY (pure yellow)
coat colour do not survive

24. 2:1 ratio from cross between
two yellow mice results
from a lethal allele.

25. Sex-linked Inheritance (x linked)

26. In the nucleus of every body cell there
are 46 chromosomes
22 homologous pair and one pair of sex
chromosomes
Normal Cells

27. Some traits are found on the sex
chromosomes instead of autosomes and are
more likely to show up in one gender.
So termed as x linked traits
These traits generally affect males more
than females.
EX: colorblindness & hemophilia
x linked

28. Haemophiliacs cannot make the blood
clotting protein Factor VIII.
It caused by a recessive allele carried on
the X but not the Y chromosome
Hence is sex-linked
Haemophilia

30. Red Green Colour Blindness
Inability to distinguish between red and
green.
A colour blind person cannot distinguish
the number clearly.
In humans normal vision is completely
dominant to colour blindness.
Heterozygous females are the carriers.

31. Genetics of Colour Blindness

32. Are you a colour blind?

33. References
Genetics – Veer Bala Rastogi
Concepts of Genetics Pearson– Klug, Cummings, et al.
Introduction to genetic analysis W. H. Freeman and Company – Griffiths, Wessler,..
Genetics - A Conceptual Approach, Benjamin A. Pierce.

34. Thank You