Alleles whose phenotypes are both expressed in the heterozygote. Example: The AB blood group is due to a single gene which produces a protein that is found on the surface of the red blood cells. The A and B alleles of this gene produce antigenically distinct forms of this protein which can be recognized by antibodies specific to each form. A person who inherits the A allele from one parent and the B allele from the other will express both forms of the protein, so their red cells will react with both the A and B antibodies. We say they have type AB blood.