Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

Co-dominance is the phenomenon in which both the alleles of a contrasting character are expressed in the heterozygous condition. Both the alleles of a gene are equally dominant. ABO blood group in human beings is an example of co-dominance. The blood group character is controlled by three sets of alleles, namely, I^A, I^B, and i. The alleles, I^A and I^B, are equally dominant and are said to be co­­-dominant as they are expressed in the AB blood group. Both these alleles do not interfere with the expression of each other and produce their respective antigens. Hence, the AB blood group is an example of co-dominance.

Allele from Parent 1	Allele from Parent 2	Genotype of offspring	Blood type of offspring
IA	IA	IA IA	A
IA	IB	IAIB	AB
IA	i	IA i	A
IB	IA	IA IB	AB
IB	IB	IBIB	B
IB	i	IB i	B
i	i	i i	O

Incomplete dominance is a phenomenon in which one allele shows incomplete dominance over the other member of the allelic pair for a character. For example, a monohybrid cross between the plants having red flowers and white flowers in Antirrhinum species will result in all pink flower plants in the F₁ generation. The progeny obtained in F₁ generation does not resemble either of the parents and exhibits intermediate characteristics. This is because the dominant allele, R, is partially dominant over the other allele, r. Therefore, the recessive allele, r, also gets expressed in the F₁ generation resulting in the production of intermediate pink flowering progenies with Rr genotype.