Homozygous

An organism is homozygous for a gene when both copies at that locus, one from each parent, are the same allele. With a dominant allele A and a recessive allele a, there are two homozygous genotypes: homozygous dominant (AA) and homozygous recessive (aa). The opposite state, with two different alleles, is heterozygous. The word applies to one locus at a time, so a person can be homozygous at one gene and heterozygous at the next.

Homozygous individuals pass the same allele to every offspring, which is why they breed true. Mendel's true-breeding pea plants, which gave the same traits generation after generation, were homozygous for the traits he studied. A homozygous recessive genotype is the only one that can be read straight from the phenotype, because the recessive trait appears only in that state.

This makes homozygous recessive individuals useful as testers. In a test cross, an organism showing the dominant trait is crossed with an aa partner; if all the offspring show the dominant trait, the unknown parent is probably AA. In medicine, a person homozygous for a disease-causing recessive variant has the condition, while a heterozygous relative is usually an unaffected carrier.