1) Variant forms of the same gene. Different alleles produce variations in inherited characteristics such as eye colour or blood type.

2) One of the alternative versions of a gene at a given location (locus) along a chromosome

3) An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. Though the term "allele" was originally used to describe variation among genes, it now also refers to variation among non-coding DNA sequences.