Allele

A genetic term that means “an alternate form of a gene.” Alleles account for genetic variation, and alleles of a gene are present at a defined location (“locus”) on a particular chromosome. For example, the ABO gene locus has three main alleles that can be inherited on chromosome 9: A, B, and O. The combination of the allele inherited from one parent with that inherited from the other determines the person’s final ABO type. For example, if a person inherits an A allele from his dad and a B allele from his mom, his ABO type will be “AB.” This can get very convoluted with multiple possible alleles, as is seen in the Rh blood group system.

The above is, of course, a simplified version of reality. As we have gained the ability to actually sequence DNA, we now know that there are NOT just three ABO alleles, for example, but literally dozens of variants of A, B, and O (see for example the list of such variants at the ISBT web site). The same is true across the spectrum of blood groups, with identification of sometimes enormous numbers of variants of what we used to think was a single allele in the past (RHD is a great example; check out the list of variants of that allele at the ISBT site).

Updated June 2024 by Joe Chaffin, MD

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