homozygosity

Homozygosity is the genetic condition in which two corresponding alleles at a given gene locus are identical. An individual who is homozygous for a locus has the same allele on both copies of the chromosome pair, while a heterozygous individual carries different alleles at that locus.

In diploid organisms, offspring inherit one allele from each parent. The level of genome-wide homozygosity is influenced by mating patterns and population history. Inbreeding or consanguinity increases homozygosity by raising the probability that both parents pass on the same ancestral allele. Population bottlenecks, genetic drift, and founder effects can also raise homozygosity by reducing overall genetic diversity.

Homozygosity can unmask recessive alleles, including deleterious mutations, leading to recessive genetic disorders and inbreeding depression. Conversely, some beneficial traits may be fixed in a population through homozygosity of advantageous recessive alleles.

In genetics research and medicine, runs of homozygosity (ROH) refer to long, contiguous stretches of homozygous loci in a genome. ROH can indicate autozygosity due to recent common ancestry and are used to study inbreeding levels, identify disease-associated regions, and map recessive genes. The inbreeding coefficient F expresses the probability that two alleles at a locus are identical by descent, and the distribution of ROH lengths can inform the timing of past inbreeding events.

In sex chromosomes, the concept differs for males in XY species, who are hemizygous for X-linked loci. The term homozygosity is most applicable to autosomal loci in diploid organisms.