heterozygot

A heterozygot, also spelled heterozygote, is an organism that carries two different alleles at a given gene locus. In sexually reproducing diploid species, one allele is inherited from each parent, so the two alleles at a locus may differ, for example Aa. This contrasts with a homozygote, in which both alleles are the same (AA or aa).

The combination of alleles constitutes the genotype, while the observable trait is the phenotype. The phenotype that arises from a heterozygous genotype depends on the relationship between the alleles: some loci show complete dominance, where the phenotype matches the dominant allele; others show incomplete dominance or semi-dominance, where the heterozygote has a blended or intermediate phenotype; and some loci exhibit codominance, where both alleles are expressed, such as in certain antigen systems.

A classic example of heterozygosity in humans is the sickle cell trait. Individuals with one normal allele (HbA) and one sickle allele (HbS) are heterozygous (HbA HbS) and usually do not have severe disease, but they can have some protective advantage against malaria in certain environments. This illustrates heterozygote advantage, where carrying two different alleles can be favored by natural selection under specific conditions.

In population genetics, heterozygosity refers to the presence of different alleles at a locus within individuals of a population and serves as a measure of genetic diversity. Higher heterozygosity generally indicates greater genetic variation, which can influence adaptability and resilience. In agriculture and breeding, heterozygotes can exhibit heterosis or hybrid vigor, where mixed-genotype individuals show improved traits compared with their parents.