Multiple Alleles:The ABO blood group in humans is determined by multiple alleles of a single gene. There are four possible phenotypes for this character: A person's blood may be either A, B, AB, or O. These letters refer to two carbohydrates--A and B-- that may be found on the surface of red blood cells. A person's blood cells may have carb. A (type A blood), carb B (type B blood), both (type AB), or neither (type O).The four blood groups result from various combinations of three different alleles for the enzyme (I) that attaches the A or B carb. to red blood cells. The enzyme encoded by the IA allele adds the A carb, whereas the IB carb adds the B carb. The enzyme encoded by the iallele adds neither A nor B. Because each person carries two alleles, six genotypes are possible, resulting in