homozygous

Homozygous describes a genetic state in which an individual carries two identical alleles for a given gene locus. This occurs in diploid organisms where each person has two copies of most genes, one inherited from each parent. A homozygous genotype is written as either AA or aa, depending on the alleles involved. By contrast, heterozygous refers to two different alleles at the same locus, such as Aa.

Phenotypic effects depend on the mode of inheritance. When a trait is governed by complete dominance, both AA and Aa individuals typically display the dominant phenotype, while aa expresses the recessive phenotype. If the recessive trait is a disease allele, only aa individuals manifest the disease; AA or Aa are typically unaffected but may be carriers if the trait is recessive.

In breeding and population genetics, the level of homozygosity influences genetic diversity. Inbreeding increases homozygosity, raising the risk of recessive disorders and reducing heterozygote advantage. Scientists study runs of homozygosity to identify regions of the genome associated with disease or to infer demographic history.

Examples help illustrate the concept. For a gene with alleles B (dominant) and b (recessive), a BB or Bb individual often shows the dominant phenotype, while bb shows the recessive phenotype. In humans, a recessive disease allele such as in cystic fibrosis requires two copies (ff) to express the condition.