Heterozygotes are zygotes (a fertilized ovum or egg) that, with regard to a specific gene at a particular chromosomal location (locus), contain different forms (alleles) of that gene. Heterozygotes develop into organisms that are heterozygous with regard to the gene in question. As a result of meiosis, heterozygotes produce gametes that are dissimilar with respect to the genes for which they are heterozygous.

In contrast to heterozygotes, homozygotes contain identical alleles of a gene at a particular locus on a chromosome.

In many cases a heterozygote has a greater degree of fitness than a homozygote for a particular gene. This advantage in fitness is termed a heterozygote advantage (also termed heterozygote superiority) and leads to a relatively stable balance (equilibrium) regarding the representation of alleles in a population (i.e., the percentage of alleles in a population). This stability also helps to preserve a variance in alleles of...