Homozygotes

A homozygote is an individual who has two identical alleles at a given genetic locus. In diploid organisms, each gene typically has two copies, and a homozygous genotype consists of either two copies of the same allele (for example, AA or aa). The contrasting genotype is heterozygous, containing two different alleles (for example, Aa).

Genotypes at a locus are often written with the alleles present, so a locus with alleles A and a can produce AA, Aa, or aa. The phenotypic effects of a homozygous genotype depend on the dominance relationships of the alleles: for a recessive trait, only homozygous recessives (aa) express the trait, while for a dominant trait, both AA and Aa may express it, depending on the trait.

Homozygosity can arise through Mendelian inheritance when both parents contribute the same allele, or more broadly through inbreeding or population structure that increases autozygosity (identical by descent). In populations with high inbreeding or genetic drift, individuals are more likely to be homozygous at many loci, which can elevate the risk of recessive genetic disorders.

Biological and medical relevance includes the fact that many inherited diseases are recessive and manifest only in homozygous individuals. In population genetics, homozygosity and heterozygosity are used to assess genetic diversity and inbreeding, often summarized by measures such as observed versus expected heterozygosity and fixation indices.