ABO blood group system
For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other ("IA", "IB") and dominant over the recessive "i" at the ABO locus. The "IA" and "IB" alleles produce different modifications. The enzyme coded for by "IA" adds an N-acetylgalactosamine to a membrane-bound H antigen. The "IB" enzyme adds a galactose. The "i" allele produces no modification. Thus the "IA" and "IB" alleles are each dominant to "i" ("IAIA" and "IAi" individuals both have type A blood, and "IBIB" and "IBi" individuals both have type B blood), but "IAIB" individuals have both modifications on their blood cells and thus have type AB blood, so the "IA" and "IB" alleles are said to be co-dominant.