- The term homozygous refers to the genetic state in which an individual carries two identical alleles at a given genetic locus on homologous chromosomes. Since diploid organisms, such as humans, inherit one copy of each chromosome from their mother and one from their father, every locus has the potential to carry two versions of a gene or DNA sequence. When both alleles at a locus are the same—whether they are both dominant or both recessive—the individual is considered homozygous at that locus. For example, a person with two alleles for blue eye color is homozygous for that trait, just as someone with two alleles for brown eyes is also homozygous, though the expressed phenotype may differ depending on whether the allele is dominant or recessive.
- Homozygosity has important implications for inheritance and phenotype. If an individual is homozygous dominant, they carry two copies of a dominant allele, and the dominant trait will be expressed consistently. In contrast, if an individual is homozygous recessive, they carry two copies of a recessive allele, and the recessive trait will only be expressed when no dominant allele is present. This distinction explains the classic Mendelian ratios observed in genetic crosses, where homozygosity plays a crucial role in determining which traits appear in offspring. For instance, pea plants with two recessive alleles for wrinkled seeds express the wrinkled phenotype, while those with two dominant alleles for round seeds express the round phenotype.
- From a broader perspective, homozygosity reflects patterns of genetic diversity in populations. While some degree of homozygosity is natural, very high levels can indicate reduced genetic variation. This often occurs in small or inbred populations, where mating between closely related individuals increases the likelihood that offspring inherit identical alleles. Such reduced variation can make populations more vulnerable to disease and environmental change, as there is less allelic diversity to provide adaptive advantages. On the other hand, certain homozygous states can be beneficial when they ensure consistency in desirable traits, which is why homozygosity is sometimes intentionally encouraged in plant and animal breeding.
- In human health, homozygosity can have critical consequences. Many genetic disorders are caused by recessive alleles that only manifest when an individual is homozygous. For example, cystic fibrosis and phenylketonuria (PKU) arise when an individual inherits two defective copies of the responsible gene. In contrast, carrying only one defective allele (heterozygous) often makes a person a symptom-free carrier. In some cases, however, being homozygous for a beneficial allele can also confer advantages, such as resistance to certain infections or increased efficiency in metabolic pathways. Thus, homozygosity can be either detrimental or advantageous, depending on the specific alleles involved.
