A son has blood type B (IBi) and his father is blood type AB. What must be the mother's genotype for the son to donate blood?

For the son to have blood type B (IBi), he must inherit one IB allele from the father and one i allele from the mother. The father's genotype, being type AB, means he can only pass on either the IA or the IB allele. Since the son has blood type B, he must have received the IB allele from his father.

This implies that the mother must provide the other allele that contributes to the son's blood type. The only alleles that can produce a blood type B child in this case from the mother's side are IB or i. If the mother were to have the genotype IAi, she could also pass on the i allele, which along with the IB allele from the father would create the son's genotype IBi. If the mother were ii, she would only pass on the i allele, which would lead to a blood type O (ii) son when combined with the IB allele, which does not match the son's type. The mother having genotype IAIB would mean she could only pass on IA, resulting in a type AB offspring if combined with the IB allele from the father.

Therefore, the correct choice points to the mother needing to have a genotype of IBi, allowing her to contribute the necessary i allele for their