Mutiple alleles (+ codominance) the ABO blood system

- Controlled by tri-allelic gene - Can generate 6 pehnotypes - The alleles control the production of the antigens on the surface of the red blood cells - Two of the alleles are codominant to one another and both are dominant over the third - Allele IA produces antigen A - Allele IB produces antigen B - Allele IO produces no antigen