It's in your blood
Question: If we came from Adam and Eve who, one would think, could have had only one blood group each, how do humans now have four blood groups—A, B, AB and O?
Answer: There is one gene in humans that controls the ABO blood type. There are three versions, or alleles, of the gene: A, B, or O. Since the gene is always present as a pair of alleles, with one inherited from each parent, two alleles are always present, so that the possible genetic make-up of any individual is AA, BB, AB, AO, BO, or OO. The 'O' allele is recessive to the A or B, meaning that in the presence of the A or B allele, the blood type is determined by the A or B allele. That is, an AO person has A-type blood; a BO person has B-type blood, whereas only an OO person has O-type blood. Anyone with O-type blood is called a universal donor because their blood lacks the A or B proteins so that O-type blood can be given to a person with A, B or AB type blood. If, for example, A-type blood is given to a person with B-type blood, an allergic reaction can result in death of the recipient.
For a husband and wife to pass on all alleles to their children, they need to, between them, have the A, B, and O alleles. So Adam and Eve could have had any of the following genetic make-ups:
AO and BO, or AB and OO, or AB and AO, or AB and BO, or AA and BO, or BB and AO.
If Adam and Eve were genetically AO and BO, for example, their children could have had AB, AO, BO or OO genetic make-up, giving AB, A, B, or O blood types. Indeed, about 25% of their children would have been of each type.
If Adam and Eve had 56 children, as per Jewish tradition, then there would have been about 14 of each blood group in Adam and Eve's family.