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.