Children born to couples with blood type A and B may have four blood types A, B, O and AB appear, due to the A, B and O blood types according to the Mendelian law of genetics. For the four blood types A, B, O and AB, in terms of genetics, type A has two genes AA and AO, and type B has two genes BB and BO. If one parent is type A, his gene is AO, and the other parent is type B, the gene is BO. By crossover, we can derive four gene types, AO, BO, OO and AB, that is, AO is blood type A, BO is blood type B, OO is blood type O, and AB gene is blood type AB, so we can derive four blood types.