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 (1:2:1)

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)
I^A	I^A	I^A I^A	A
I^A	I^B	I^A I^B	AB
I^A	i	I^Ai	A
I^B	I^A	I^A I^B	AB
I^B	I^B	I^B I^B	B
I^B	i	I^Bi	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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