My brother is Bo, his wife A_, and their son is A_...would not Aa or Ao x Bo produce either AB or O? What must the silent type of the mother be in the case of Bo x A_ to produce an A_? Could the baby be Ab or Ao...is there such a thing as Ab or is it always AB..or what? Thanks, John -- in Calgary, Canada

Hi, John! I assume you know your brother is B(o) because (1) he tested as type B, and (2) one of your parents was type B or AB, and the other type O.

I think your question stems from the idea of A and B both being dominant to O. This is true in a single individual, but a parent can donate either of her genes to any child.

Therefore, your brother can donate an O gene (his recessive one) to the child, and his wife can donate an A gene (her dominant one). There you are: a type A kid.

Your brother and his wife may be the classic ABO wildcard combination: B(o) + A(o). Couples with this ABO configuration can have type A(o), B(o), O or AB children. OR, they might be B(o) + A(a), in which case their offspring can only be type A(o) or type AB. In either case, a type A child is certainly possible in this pairing no matter what the silent type, or genotype, of the type A mother.

"AB" is the standard name for that blood type, but if it were "BA" instead, it would make no difference. The two letters are both capitalized because the A and B genes are co-dominant; neither is recessive to the other.

Hope this clears it up!