Multiple allelism describes genes that exist in three or more allelic forms. Although diploid organisms, like humans, normally possess only two alleles of each gene, there are multiple alleles of many (if not most) human genes present in a population. Blood type is one example of multiple allelism. There are three alleles for blood type (HBB gene) in humans: IA, IB, and i.
Sickle cell anemia, which is caused by a mutation in the gene encoding beta-globin (HBB), is one example of incomplete dominance. Two copies of the sickle cell allele are required for the disease, with sickle cell homozygotes producing stiff, crescent-shaped red blood cells that clog blood vessels. On the other hand, individuals homozygous for the normal beta-globin allele make flexible, disc-like erythrocytes that travel easily through the vasculature.
However, heterozygotes that have one normal allele and one sickle cell allele make both normal (disc-shaped) and sickle-shaped red blood cells, and are said to possess the sickle cell trait. These individuals rarely suffer from complications of the disease, unless, for example, they encounter low oxygen levels. This is an example of incomplete dominance, since a heterozygote exhibits an intermediate phenotype between that of healthy and sickle-shaped cells.
At the molecular level, sickle cell heterozygotes also exhibit codominance, since the normal and sickle cell alleles produce roughly the same levels of their protein products in red blood cells.
Blood type is another example of codominance. IA, IB, and i are all blood type alleles in humans. IA and IB alleles encode A and B antigen proteins, respectively, and the i allele encodes no antigen protein at all. IA and IB are dominant over i, and two copies of i are necessary for the blood type O phenotype. However, IA and IB are codominant. Thus, in an IAIB heterozygous individual, both A and B antigens are expressed and found on the surface of each red blood cell.
