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

Other Patterns of Inheritance (Non-Mendelian Inheritance)

1. Incomplete Dominance

It is an inheritance in which heterozygous offspring show an intermediate character between two parental characteristics.
E.g., flower colour in snapdragon (dog flower or Antirrhinum sp.) and Mirabilis jalapa (4’O clock plant).

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

It is the inheritance in which both alleles of a gene are expressed in a hybrid.
E.g., ABO blood grouping in humans.
ABO blood groups are controlled by the gene I.
This gene controls the production of sugar polymers (antigens) that protrude from the plasma membrane of RBC.
The gene I has three alleles: I^A, I^B, & i.
I^A and I^B 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^AI^A	A
I^A	I^B	I^AI^B	AB
I^A	i	I^Ai	A
I^B	I^A	I^AI^B	AB
I^B	I^B	I^BI^B	B
I^B	i	I^Bi	B
i	i	ii	O

When I^A and I^B are present together, they both express their own types of sugars. This is due to co-dominance.

It is the presence of more than two alleles of a gene to govern the same character.
E.g., ABO blood grouping (3 alleles: I^A, I^B, & i).
In an individual, only two alleles are present. Multiple alleles can be found only in a population.

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 the 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 are responsible for dark skin colour, and recessive forms a, b, & c for light skin colour.
- Genotype with all the dominant alleles (AABBCC) gives the darkest skin colour.
- Genotype with all the recessive alleles (aabbcc) gives the lightest skin colour.
- Therefore, a genotype with 3 dominant alleles and 3 recessive alleles gives an intermediate skin colour.
- Thus, the number of each type of allele determines the darkness or lightness of the skin.

Here, a single gene exhibits multiple phenotypic expressions. Such a gene is called a 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 the BB gene. Therefore, large starch grains are produced.
- bb has lesser efficiency in starch synthesis and produces smaller starch grains.
- Starch grain size also shows incomplete dominance.

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