homocigosis

Homocigosis, or homozygosity, is the genetic state in which an individual has two identical alleles at a given locus on paired chromosomes. This contrasts with heterozygosity, where the alleles differ. An organism can be homozygous for a gene with either a dominant or a recessive allele: homozygous dominant (for example, AA) and homozygous recessive (aa). The phenotype that results depends on the gene and its mode of inheritance; a recessive trait is expressed only when the individual is homozygous for the recessive allele, while a dominant trait can be expressed in both homozygous dominant (AA) and heterozygous (Aa) individuals, with the term homozygous referring specifically to having two identical copies.

Homozygosity arises when a zygote inherits the same allele from both parents. This can occur through mating between related individuals, which increases the chance that both parents carry the same allele, or in populations with a small effective size or strong selection for particular alleles. Population genetics uses measures such as runs of homozygosity to assess autozygosity and recent inbreeding.

In breeding and research, homozygous lines are valued for producing uniform offspring and stable traits across generations. In medicine, homozygosity for pathogenic recessive alleles underlies many autosomal recessive disorders, such as cystic fibrosis or sickle cell disease, while heterozygotes may be carriers without full disease expression. Understanding homozygosity helps illuminate inheritance patterns, disease risk, and strategies in genetics and breeding.