The genotype for blood type A+ is AA or AO. The ABO blood group system is determined by the presence or absence of antigens A and B on the surface of red blood cells, as well as the presence or absence of antibodies against the opposite antigens in the plasma. In the ABO system, A and B are co-dominant, while O is recessive. Therefore, a person with blood type A can have either two copies of the A antigen (genotype AA) or one copy each of the A and O antigens (genotype AO). The Rh factor, which is either positive or negative, is determined by a separate gene and is not related to the ABO blood group system. People with blood type A+ have the A antigen on the surface of their red blood cells and also have the Rh factor antigen (positive).