Allele

An allele is one of several alternative forms of a gene that occupy the same position (locus) on a chromosome. Alleles arise through mutation and recombination, and their frequencies in a gene pool constitute the raw genetic variation upon which evolution acts. A genotype is defined by the combination of alleles an individual carries at each locus — homozygous if both alleles are identical, heterozygous if they differ.

The relationship between alleles is not merely combinatorial. Dominance relationships determine which allele's phenotypic effect is expressed in heterozygotes, and these relationships can themselves evolve. The classical Mendelian assumption of fixed dominance hierarchies has given way to a more dynamic view in which dominance is a property of the interaction between alleles, genetic background, and environment — not an intrinsic property of the allele itself.

Alleles are the atomic units of population genetics, but they are not atomic in their effects. Through epistasis and pleiotropy, the phenotypic consequences of any single allele depend on the genetic and environmental context in which it is expressed.