A father has blood group A and mother has blood group B. What will be…

The correct answer is Option (2) → A, B, AB and O

ABO blood groups are controlled by gene I with three alleles, I^A, I^B and i. I^A and I^B are dominant over i and co-dominant with each other. A father of group A can be I^Ai, and a mother of group B can be I^Bi. The question asks what is possible, so take both as heterozygous:

	IB (mother)	i (mother)
IA (father)	IAIB = AB	IAi = A
i (father)	IBi = B	ii = O

All four blood groups can appear among the children.

Option (1) AB — Incorrect. AB arises only when both parents pass on I^A and I^B. It is one possible group, but not the only one.

Option (2) A, B, AB and O — Correct. With heterozygous parents, the children can be A, B, AB or O, as the square shows.

Option (3) Only O — Incorrect. Group O needs i from both parents. It is possible, but the other three groups are possible as well.

Option (4) Only A and B. — Incorrect. A and B are possible, but AB and O are left out, so “only A and B” is wrong.

NCERT Extract:
Co-dominance – “… The gene (I) has three alleles I^A, I^B and i. The alleles I^A and I^B produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. I^A and I^B are completely dominant over i, in other words when I^A and i are present only I^A expresses (because i does not produce any sugar), and when I^B and i are present I^B expresses. But when I^A and I^B are present together they both express their own types of sugars: this is because of co-dominance.”

Table 4.2: Table Showing the Genetic Basis of Blood Groups in Human Population