An allele is one of a number of alternative forms of the same gene or same genetic locus. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, most genetic variations result in little or no observable variation.

Most multicellular organisms are diploid. This means that they possess two alleles for each gene, one inherited from each parent. If the two alleles are the same, the organism is said to be homozygous with respect to that gene. If they are different we call the organism heterozygous with respect to that gene.