An allele (pronounced /ˈæliːl/ (UK), /əˈliːl/ (US); from the Greek αλληλος allelos, meaning each other) is one of two or more forms of the DNA sequence of a particular gene.

Each gene can have different alleles. Sometimes, different DNA sequences (alleles) can result in different traits, such as color. Sometimes, different DNA sequences (alleles) will have the same result in the expression of a gene.

Most organisms have two sets of chromosomes, that is, they are diploid. Diploid organisms have one copy of each gene (and one allele) on each chromosome. If both alleles are the same, they are homozygotes. If the alleles are different, they are heterozygotes.

A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles (polymorphism) present, or the proportion of heterozygotes (heterozygosity) in the population.

The word is a short form of allelomorph ('other form'), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes.

For example, at the gene locus for ABO blood type proteins in humans, classical genetics recognizes three alleles, I, I, and I, that determines compatibility of blood transfusions. Any individual has one of six possible genotypes (AA, AO, BB, BO, AB, and OO) that produce one of four possible phenotypes: "A" (produced by AA homozygous and AO heterozygous genotypes), "B" (produced by BB homozygous and BO heterozygous genotypes), "AB" heterozygotes, and "O" homozygotes. It is now appreciated that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus. An individual with "Type A" blood may be a AO heterozygote, an AA homozygote, or an A'A heterozygote with two different 'A' alleles.