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An allele is a variant or alternative form of a given gene at a locus. As an example, ash red and brown are alleles at the b locus. Genes at different loci are not alternative forms of a single gene, and therefore are not alleles. For example, indigo and recessive opal are not alleles.
Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation or morphology. The word "allele" is a short form of "allelomorph".