Mr. Andrew, his wife and child belong to blood groups A, B and O respectively. The genotypes of both parents are

Analyze the question: Mr. Andrew is blood group A, his wife is group B, and their child is group O. We need the parents' genotypes.

Concept: The ABO blood group system is controlled by three alleles: IA, IB and IO. IA and IB are codominant, and both are dominant over IO.

• Group A: IAIA or IAIO
• Group B: IBIB or IBIO
• Group AB: IAIB
• Group O: IOIO

Solution:

1. The child is group O, so the child's genotype must be IOIO.
2. The child gets one IO allele from each parent. So both parents must carry at least one IO allele.
3. Father (group A) must therefore be IAIO, not IAIA.
4. Mother (group B) must therefore be IBIO, not IBIB.

Check with a Punnett square (IAIO × IBIO):

| | IB | IO | |---|---|---| | IA | IAIB (AB) | IAIO (A) | | IO | IBIO (B) | IOIO (O) |

This cross can produce a group O child, which matches the question.

Checking the other options:

• A and B: one parent is IBIB or both are homozygous, so there is no IO allele from that parent and a group O child is impossible.
• C: IAIB is group AB, not group A or B, so it does not fit the parents' groups.

Answer: D. The father is IAIO and the mother is IBIO.

Tip: Whenever a group O child appears, both parents must carry the recessive IO allele. Start with the O child's genotype (IOIO) and work backwards.