Noun, pl. alleles
1. One of a number of alternative forms of the same gene or same genetic locus (generally a group of genes). It is the alternative form of a gene for a character producing different effects. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, many variations at the genetic level result in little or no observable variation. (wikipedia.org)
One member of a pair (or any of the series) of genes occupying a specific spot on a chromosome (called locus) that controls the same trait.