Codominance in ABO blood group.
<P>
   The A, B, AB, or O phenotypes are affected by 3 different alleles.
<P>
   I^A  and  I^B alleles produce different antigens on the surface of red
   blood  cells,  thus  are  dominant  to  the i allele which produces no
   antigen.
<P>
   I^A  and  I^B  are codominant to each other because the RBCs bear both
   antigens.