Multiple Alleles

Two human examples of multiple-allele genes are the gene of the ABO blood group system, and the human-leukocyte-associated antigen (HLA) genes.

The ABO system in humans is controlled by three alleles, usually referred to as I^A, I^B, and I^O (the "I" stands for isohaemagglutinin). I^A and I^B are codominant and produce type A and type B antigens, respectively, which migrate to the surface of red blood cells, while I^O is the recessive allele and produces no antigen. The blood groups arising from the different possible genotypes are summarized in the following table.

HLA genes code for protein antigens that are expressed in most human cell types and play an important role in immune responses. These antigens are also the main class of molecule responsible for organ rejections following transplantations—thus their alternative name: major histocompatibility complex (MHC) genes.

The most striking feature of HLA genes is their high degree of polymorphism—there may be as many as one hundred different alleles at a single locus. If one also considers that an individual possesses five or more HLA loci, it becomes clear why donor-recipient matches for organ transplantations are so rare (the fewer HLA antigens the donor and recipient have in common, the greater the chance of rejection).