What Is an Allele?

An allele is one of two or more versions of a gene that exist at the same location on a chromosome. Since a person inherits one copy of each gene from each parent, they typically have two alleles for every gene — one maternal and one paternal.

Alleles can be identical (homozygous) or different (heterozygous). Differences between alleles account for much of human genetic variation, including physical traits and, in some cases, disease risk. For example, different alleles of a single gene can determine blood type or influence the likelihood of developing certain hereditary conditions.

References: medlineplus.gov/genetics; genome.gov