Monohybrid Inheritance continued

Codominance and Incomplete dominance

Codominance and incomplete dominance are both types of non-Mendelian inheritance patterns that involve the expression of alleles in a way different from classical Mendelian genetics.

Codominance

In codominance, both alleles at a gene locus are fully expressed in the heterozygous condition.

This means that neither allele is dominant or recessive, and both contribute to the phenotype.

The ABO blood group system is a classic example of codominance. In this system, individuals can have blood types A, B, AB, or O, based on the presence or absence of two antigens (A and B) on the surface of red blood cells, as well as the type of antibodies in their plasma.

Let’s consider the alleles involved in blood types:

IA (allele for blood type A):

Individuals with two copies of this allele (AA) have blood type A.

The A allele codes for the production of A antigens on the surface of red blood cells.

IB (allele for blood type B):

Individuals with two copies of this allele (BB) have blood type B.

The B allele codes for the production of B antigens on the surface of red blood cells.

IA and IB (heterozygous genotype):

Individuals with one copy of the A allele and one copy of the B allele (AB) have blood type AB.

This is the co-dominant condition where both A and B antigens are expressed on the surface of red blood cells.

i (allele for blood type O):

Individuals with two copies of the O allele (ii) have blood type O.

The O allele does not produce A or B antigens on the surface of red blood cells.

Punnett Square for Blood Types:

To represent codominance in a Punnett square, uppercase letters (IA and IB) are used to denote the alleles. Let’s construct a Punnett square for the cross between individuals with blood types A (IAIA) and B (IBIB). 

In this Punnett square, you can see that the IA and IB alleles are both expressed in the heterozygous condition (IAIB), resulting in blood type AB.

Phenotypes:

AA (homozygous for A) → Blood type A

BB (homozygous for B) → Blood type B

AB (heterozygous) → Blood type AB (co-dominant expression)

ii (homozygous for O) → Blood type O

Incomplete Dominance

In incomplete dominance, the phenotype of the heterozygous individual is an intermediate or blend of the phenotypes of the homozygous individuals.

There is a partial expression of both alleles in the heterozygous condition.

A classic example is the flower colour in snapdragons. If red (RR) and white (WW) are the pure-breeding parents, the heterozygous condition (RW) results in pink flowers.

In a Punnett square, incomplete dominance is represented by a blending of traits in the heterozygous individual.

• RR (homozygous for red) → Red flowers

• WW (homozygous for white) → White flowers

• RW (heterozygous) → Pink flowers (incomplete dominance)

Co-dominance involves the equal expression of both alleles, while incomplete dominance results in an intermediate phenotype in the heterozygous condition.