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Multiple Allele Traits

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For the same gene multiple alleles can interact to influence phenotypes like the shape and protein composition of an individual cells.

For example in erythrocytes, or red blood cells the hemoglobin protein has a component encoded by the beta-globin gene for which several different alleles have been identified. Collectively these are the multiple alleles of beta-globin.

For the mutant sickle allele, homozygotes demonstrate stiff, crescent shaped red blood cells that clog vessels, whereas those with the normal allele exhibit flexible disc like erythrocytes that travel easily through the vasculature. Interestingly heterozygotes may display some mildly curved cells, a shape exacerbated under low oxygen.

Since the erythrocyte morphology of heterozygotes falls between that of the homozygotes this is an example of an allele interaction called incomplete dominance. If the sickle allele were dominant heterozygotes would only demonstrate sickle shaped cells which isn't the case.

Molecularly heterozygotes showcase another allele interaction called codominance. This is because the protein products of both the normal allele and sickle allele, which causes hemoglobin proteins to stick together occur in roughly equal quantities in their red blood cells. If only the sickle allele were expressed it would be dominant. However this doesn't occur.

By studying allele interactions on the molecular and cellular levels researchers can understand the resulting phenotypes and complications of human conditions like sickle cell trait, improving treatment.

Overview

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The Concept of Multiple Allelism

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: I^A, I^B, and i.

Incomplete Dominance

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.

Codominance

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. I^A, I^B, and i are all blood type alleles in humans. I^A and I^B alleles encode A and B antigen proteins, respectively, and the i allele encodes no antigen protein at all. I^A and I^B are dominant over i, and two copies of i are necessary for the blood type O phenotype. However, I^A and I^B are codominant. Thus, in an I^AI^B heterozygous individual, both A and B antigens are expressed and found on the surface of each red blood cell.

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