Final answer:
When two parents with AB blood type have children, the possible phenotypes of their offspring can be 4AB, 2A, and 1B.
Step-by-step explanation:
When two individuals with AB blood types have children, the children can have the phenotypes A, B, or AB. This is because the ABO blood groups in humans are controlled by the IA, IB, and i alleles, with IA encoding the A blood group antigen, IB encoding the B blood group antigen, and i encoding the O blood group antigen. Both the IA and IB alleles are dominant to the i allele. So, if two parents with AB blood type (1A1B) have offspring, the possible phenotypes of their children can be 4AB, 2A, and 1B.