The I gene can add molecules called antigens to the surface of red blood cells.

The I^A and I^B alleles are codominant because they add different antigens.

The i allele is recessive because it does not add antigens.

4 phenotypes from the 3 alleles are possible:

• Type A is either I^AI^A or I^Ai.
• Type B is either I^BI^B or I^Bi.
• Type AB is I^AI^B.
• Type O is ii.

Exercise: