Any of the possible forms in which a gene for a specific trait can occur: In almost all animal cells, two alleles for each gene are inherited, one from each parent. Paired alleles (one on each of two paired chromosomes) that are the same are called "homozygous", and those that are different are called "heterozygous".

In heterozygous pairings, one allele is usually dominant, and the other recessive. Complex traits, such as height and longevity are usually caused by the interactions of numerous pairs of alleles, while simple traits, such as eye color may be caused by just one pair.

Alleles (genes that are responsible for alternative characteristics) hypothesized to encode the production of a identification cue: In addiction, an allelic recognition can simultaneously identify the cue in others, leading to the discrimination of kin from non-kin.