Children born of blood type A and blood type B may have blood types A, B, AB and O. The blood type of a person is inherited from the parents and remains unchanged for life. One parent is type A and one is type B. The type A may be purely AA or heterozygous AO, and the type B may likewise be purely BB or heterozygous BO, where O is hidden, that is, not expressed. When parents pass on their blood types to their children, the A parent may provide A or O, and the B parent may provide B or O. Each parent provides one, and then the two combine to form the blood type of the child. Therefore, the child may be AB, AO, BO or OO, with AB being AB, AO being A, BO being B and OO being O. Therefore, all four blood types of the child are possible.