Sunday, July 26, 2020

Principles of Inheritance and Variation - Notes | Class 12 | Part 4: Other Patterns of Inheritance

5. PRINCIPLES OF INHERITANCE AND VARIATION

OTHER PATTERNS OF INHERITANCE (NON-MENDELIAN INHERITANCE)

1. Incomplete Dominance


- It is an inheritance in which heterozygous offspring shows intermediate character b/w two parental characteristics.

- E.g. Flower colour in snapdragon (dog flower or Antirrhinum sp.) and Mirabilis jalapa (4’O clock plant).


Here, cross between homozygous red & white produces pink flowered plant. Thus phenotypic & genotypic ratios are same.

Phenotypic ratio= 1 Red: 2 Pink: 1 White 

Genotypic ratio=  1 (RR): 2 (Rr): 1(rr)

This means that R was not completely dominant over r.

-    Pea plants also show incomplete dominance in other traits.


2. Co-dominance


- It is the inheritance in which both alleles of a gene are expressed in a hybrid. E.g. ABO blood grouping in human.

-  ABO blood groups are controlled by the gene I.

-  This gene controls the production of sugar polymers (antigens) that protrude from plasma membrane of RBC.

-  The gene I has three alleles IA, IB & i.

-  IA and IB produce a slightly different form of the sugar while allele i doesn’t produce any sugar.


Alleles from parent 1

Alleles from parent 2

Genotype of offspring

Blood types (phenotype)

IA

IA

IA IA

A

IA

IB

IA IB

AB

IA

i

IAi

A

IB

IA

IA IB

AB

IB

IB

IB IB

B

IB

i

IBi

B

i

i

ii

O

When IA and IB are present together, they both express their own types of sugars. This is due to co-dominance.


3. Multiple allelism


-    It is the presence of more than two alleles of a gene to govern same character.

-    E.g. ABO blood grouping (3 alleles: IA, IB & i).

-    In an individual, only two alleles are present. Multiple alleles can be found only in a population.


4. Polygenic inheritance


-    It is the inheritance in which some traits are controlled by several genes (multiple genes).

-    E.g. human skin colour, human height etc.

-    It considers the influence of environment.

-   In a polygenic trait, the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.


Human skin colour:

-  Assume that 3 genes A, B, C control human skin colour. The dominant forms A, B & C responsible for dark skin colour and recessive forms a, b & c for light skin colour.

-  Genotype with all the dominant alleles (AABBCC) gives darkest skin colour.

Genotype with all the recessive alleles (aabbcc) gives lightest skin colour.

Therefore, genotype with 3 dominant alleles and 3 recessive alleles gives an intermediate skin colour.

-  Thus, number of each type of alleles determines the darkness or lightness of the skin.


5. Pleiotropy


-  Here, a single gene exhibits multiple phenotypic expressions. Such a gene is called pleiotropic gene.

- In most cases, the mechanism of pleiotropy is the effect of a gene on metabolic pathways which contributes towards different phenotypes.

-  E.g. Starch synthesis in pea, sickle cell anaemia, phenylketonuria etc.

- In Phenylketonuria & sickle cell anaemia, the mutant gene has many phenotypic effects. E.g. Phenylketonuria causes mental retardation, reduction in hair and skin pigmentation.


Starch synthesis in pea plant:


-    Starch is synthesized effectively by BB gene. Therefore, large starch grains are produced. bb have lesser efficiency in starch synthesis and produce smaller starch grains.

-    Starch grain size also shows incomplete dominance.


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