A son has blood type A (IAi) and his father is blood type AB. Which of the following is not a possible genotype for the mother?

The correct answer reflects the scenario in which the son has blood type A with the genotype IAi and the father has blood type AB, which is represented by the genotype IAIB. To determine the possible genotypes for the mother, we need to consider how these blood types can combine.

For the son to have a blood type A genotype of IAi, he must inherit one IA allele from his mother. Since the father can only contribute either IA or IB, the mother's genotype must have the ability to provide an i (which corresponds to blood type O) since the father does not contribute an i.

If the mother's genotype were IAIB, she would also contribute an IA allele, resulting in potential offspring with either blood type A (IAIA or IAi) or blood type B (IBi), but not blood type A with the combination of an i from the mother, which is essential in this case.

Thus, the genotype of IAIB cannot result in a child with blood type A (IAi) if paired with an AB father because it lacks the necessary i allele for that type. Hence, the genotype of the mother that does not allow for the possibility of the son having blood type A is IAIB.