Allele

An allele is one of the alternative forms of a gene or other DNA sequence at a particular position, or locus, on a chromosome. Alleles arise by mutation and may differ by a single base or by a longer stretch of DNA. Because humans and many other organisms are diploid, each individual carries two alleles at every autosomal locus, one inherited from each parent.

An individual carrying two identical alleles at that locus is homozygous; one carrying two different alleles is heterozygous. How the two alleles interact decides the trait. A dominant allele shows its effect even in a single copy, a recessive allele only when two copies are present, and with codominance, as in the human ABO and MN blood groups, both alleles are expressed. Although one person has at most two alleles of a gene, a population can hold many, and the most common form is often called the wild type.

In genetics problems, alleles are written as letters, a capital for the dominant allele and lowercase for the recessive one, as in Aa. Clinical geneticists often speak of variants rather than alleles when describing DNA changes linked to disease.