An allele is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

Alleles can differ at a single position through single-nucleotide polymorphisms, but they can also have insertions and deletions of up to several thousand base pairs. Most alleles result in little or no change in the characteristics of an individual organism but sometimes different alleles can result in different observable phenotypic traits such as antibiotic resistance