On the other hand, since his erythrocytes contain neither A nor B and so can't make trouble, he can give his blood to a person of any blood type. He is a universal donor. Sometimes it is only necessary to give the patient plasma, and that makes things simpler. It is generally true, then, that plasma can be transferred from any donor to any patient. Whether a person is of blood type A, B, AB, or O depends on the nature of the genes of that series which he has inherited.
Every person has two genes of that gene series, one on each of a pair of chromosomes.
He inherits one gene from his father and one from his mother. Three types of homozygotes are possible among these blood groups. A person can have a B gene on both chromosomes or an O gene. The man will produce sperm cells which will all carry the A gene. The woman will produce egg cells that will all carry the O gene.