A woman with type A blood has two children, one with O and one with B blood. What type of blood must the father have? Give the genotypes of the...

Blood type is determined by what antigen (protein) type is found on the surface of red blood cells. Type A people have A antigens on their red blood cells, type B have B antigens, people with type AB blood have both, and those with type O have neither. Your question is about the genotypes involved. Type A individuals may have either AA or AO genotype. Type B can have BB or BO. Those with AB...

Blood type is determined by what antigen (protein) type is found on the surface of red blood cells. Type A people have A antigens on their red blood cells, type B have B antigens, people with type AB blood have both, and those with type O have neither. Your question is about the genotypes involved. Type A individuals may have either AA or AO genotype. Type B can have BB or BO. Those with AB have received the allele for A from one parent and for B from the other (and their genotype is AB), and type O have received two copies of the allele for O (genotype OO).

Because the woman's children are type O and type B and the mother has type A, we know that the mother's genotype is AO; she has given the O gene to both children, as neither has A antigens. One child is type O, and therefore received the allele for O from both parents. We can therefore determine that one of the alleles in the father genotype is also for the O antigen. Because the other child is type B and received an O allele from the mother, the B must come from the father; the father is genotype  BO, and he has type B blood.

In sum: mother is type A blood, genotype AO.  Father is type B, genotype BO. Child 1 is type O, genotype OO. Child 2 is type B, genotype BO.