Co-dominance and Multiple Alleles — ABO Blood Groups

Co-dominance is a second important deviation from simple Mendelian dominance, and it is worth distinguishing carefully from incomplete dominance (§4.5), with which it is frequently and easily confused. In co-dominance, BOTH alleles present in a heterozygous individual are expressed fully, simultaneously, and independently in the observable phenotype — neither allele masks the other's expression (unlike complete dominance) and neither allele blends with the other into some averaged, intermediate form (unlike incomplete dominance); instead, both parental phenotypes appear together, side by side and equally distinctly, in the very same heterozygous individual.

The single best-known, and medically by far the most important, human example of co-dominance is the ABO blood group system, first worked out by the Austrian physician Karl Landsteiner (a discovery for which he was later awarded the Nobel Prize, and one that made safe blood transfusion possible for the first time). The ABO blood group is also the classic textbook example of multiple allelism: a single gene, conventionally called the I gene (for isoagglutinogen), exists in a human population in three distinct allelic forms — IA, IB and i — even though any single individual, being diploid, can carry at most two of these three alleles at once (one on each of their two homologous chromosome 9 copies, where the gene is located). The relationships among these three alleles are genuinely instructive because they combine two DIFFERENT modes of dominance within a single gene: both IA and IB are completely dominant over the third allele, i, which is fully recessive and produces no functional antigen at all on the surface of the red blood cell — so a person with genotype IAIA or IAi has blood group A (only the A antigen present), a person with genotype IBIB or IBi has blood group B (only the B antigen present), and a person with genotype ii has blood group O (neither antigen present, because the recessive i allele's product is functionally absent). But when a person's genotype is IAIB — one copy of each of the two "strong" alleles, and no copy of i — that person's red blood cells display BOTH the A antigen AND the B antigen on their surface simultaneously and in full, undiminished amount, giving blood group AB: here IA and IB are said to be co-dominant with each other, since neither is able to mask or suppress the other's expression at all. …