Allele. Definition. noun, plural: alleles. (genetics). 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. Supplement. For example, a pair of alleles controlling the same trait, i.e. eye color: one allele codes for blue eyes, another allele for brown eyes. In humans, simple traits such as eye color may be caused by the interaction of only one pair of alleles.