An example of codominance is the ABO blood groups. The A and B alleles are dominant over the O allele; the O allele is recessive. Neither the A nor the B allele, however, is dominant over the other.
Humans can have one of four blood types: A, B, AB and O. Blood type is governed by two alleles, or forms of genes, one coming from each parent. Individuals with type A blood have alleles AA or AO. People with type B blood have alleles BB or BO. Type AB blood people can only have the alleles A and B. People with type O blood have two O alleles.
In type AB blood, both the A and B alleles are codominant. This means that neither allele can mask the other, so both are expressed. Either allele will mask the O allele because it is recessive. Type A blood produces A antigen, an identifying substance on red blood cells. Type B blood produces B antigen. Individuals with AB blood produce both types of antigen.