Question

Explain the phenomena of dominance, multiple allelism and co-dominance taking human ABO blood group as an example.

Answer 1

The inheritance of ABO blood groups in humans exhibits dominance, codominance, and multiple allelism. In humans, the ABO blood groups are controlled by a gene called gene ‘I’. It has three alleles, namely I^A, I^B and i.

Dominance: The alles I^A and I^B are dominant over the allele i. I^A and I^B produce antigens A and B respectively, however allele i does not produce any antigen.

Co-dominance: If both I^A and I^B are present in an individual, then they both are expressed because of the phenomenon of co-dominance.

Multiple allelism: Since the blood grouping is governed by more than two alleles, it is a good example of multiple allelism.

Table Showing the Genetic Basis of Blood Groups in Human Population is given as follows:

Allele from Parent 1	Allele from Parent 2	Genotype of offspring	Blood-types of offspring
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	i i	O