Heterozygotes

An organism is heterozygous at a given genetic locus when it carries two different alleles, for example Aa. In diploid species, this contrasts with homozygous individuals that carry two copies of the same allele (AA or aa).

For a typical dominant-recessive trait, the phenotype of a heterozygote is usually the same as that of the dominant homozygote, because the dominant allele masks the recessive one. But heterozygotes do not always produce a single phenotype: in codominance, both alleles are expressed (as in AB blood groups), and in incomplete dominance, the phenotype is intermediate (e.g., red x white flowers yielding pink offspring).

Heterozygotes are central to population and evolutionary genetics. Some heterozygotes enjoy higher fitness than either homozygote, a phenomenon known as heterozygote advantage or balancing selection. The classic human example is the sickle cell trait (one HbA and one HbS allele), which confers malaria resistance in certain environments.

In population genetics, allele and genotype frequencies follow Hardy-Weinberg expectations (2pq for heterozygotes) under certain assumptions. In breeding and conservation, maintaining heterozygosity can preserve genetic diversity and adaptive potential.

Detection of heterozygotes typically relies on molecular genotyping or sequencing, though pedigree analysis can reveal heterozygosity in families. Conceptually, heterozygosity refers to having two different alleles at a locus, a common state in sexually reproducing populations.