Red blood cells have many types of molecules sticking out of their cell membranes. Some of these are glycoproteins that determine your blood type.

The genes that code for these glycoproteins exhibit something called "co-dominance" where I^A and I^B are both equally dominant but O is still recessive.

The genotype for blood type A is written as I^AI^A or I^Ai (where "i" is recessive). Blood type B is I^BI^B or I^Bi and blood type O is ii.

But put I^A and I^B together and you get another blood type, AB, the co-dominant type, where neither the A or B is recessive to the other.

A father has blood type A and a mother has blood type AB. They have 3 children, a son with blood type A, another son with blood type AB, and a daughter with blood type B. The daughter then marries a man with blood type O and they have a daughter with type O.