Aug 20, 2014  Lets start with a quick review of blood types. Recall that for blood types, A and B refer to the presence of the A and B antigens, respectively, present on red blood cells. Type A red blood cells express the A antigen; Type B red blood cells express the B antigen. Type O blood expresses neither A nor B antigen. That means two genotypes can code for blood that is phenotypically type A: I A I A or I A