Final answer:
When one parent has blood type A and the other parent has blood type B, their children can have blood types A, B, AB, or O.
Step-by-step explanation:
When determining the possible blood types of offspring, it is important to understand the inheritance of ABO blood group alleles. If one parent has blood type A (genotype IAIA or IAi) and the other parent has blood type B (genotype IBIB or IBi), their children can have blood type A, blood type B, blood type AB, or blood type O. Therefore, it is possible for a child with blood type O (genotype ii) to be a legitimate child of parents in which one parent has blood type A and the other parent has blood type B.