Human ABO blood groups are determined by three alleles: IA, IB and i. A man with blood group AB has a number of children by a woman with blood group A. Their first child has blood group B. What is the probability that the second child will also have blood group B?
A) 1/2
B) 1/4
C) 1/8
D) 1/16
E) 3/4

Question

Quizplus · Accepted Answer

The man with blood group AB has genotype IAIB. The woman with blood group A can have genotype IAIA or IAi. For the first child to have blood group B, the woman must have genotype IAi. The possible genotypes for their children are IAIA, IAi, IBIA, and IBi. Only IBIA and IBi result in blood group B, giving a probability of 1/4.

Human ABO Blood Groups Are Determined by Three Alleles: IA