Final answer:
When a man with genotype aa and a woman with genotype bb have children, their offspring can have the A, B, or AB blood type.
Step-by-step explanation:
The ABO blood type is determined by the presence or absence of antigens on the surface of red blood cells. In this case, the man has genotype aa (blood type A with no antigens) and the woman has genotype bb (blood type B with no antigens).
When two individuals with the genotype aa and bb mate, their children will inherit one allele from each parent. This means that their children can have a genotype of ab (blood type AB, with antigens from both parents), or either a or b (blood type A or B, with antigens from only one parent).
The possible phenotypes of their offspring in terms of ABO blood group are AB, A, or B.