An allele is one of two or more variants in the DNA sequence of a gene. For instance, the gene for eye colour in humans has a number of DNA sequence variants and each of these variants is a different allele for eye colour. In some cases different alleles of a gene can have different phenotypic manifestations, as in the case of the allele for brown eyes versus the allele for blue eyes, but often there is no phenotypic difference between two alleles of a gene. An allele is one example of a genetic polymorphism.

Most organisms are diploid, carrying two alleles for each gene (each allele occupies an identical locus on homologous chromosomes). If the two alleles carried are identical then the organism is said to be homozygous; if the two alleles are different then the organism is heterozygous. The two alleles interact with each other to generate the phenotype of the cell in which they're found; the way in which they interact depends on their dominance relationship.

The frequency of all the alleles of all the genes in a population constitutes the gene pool of that population. Changes in the gene pool, via changes in allele frequency, are the basis of evolution.