Heterozygote

Heterozygote, in genetics, refers to an organism or cell that has two different alleles at a given genetic locus on paired chromosomes. For a gene with two alleles A and a, an individual with genotype Aa is heterozygous, whereas AA or aa are homozygous. In diploid organisms, each cell contains two copies of each chromosome, and thus two alleles per gene. A heterozygous state does not imply multiple genes, just a difference at that particular locus.

The effect of heterozygosity on phenotype depends on the relationship between alleles. In complete dominance, the phenotype reflects the dominant allele, and the recessive allele is not expressed. In incomplete dominance, the phenotype is intermediate between the two alleles. In codominance, both alleles contribute to the phenotype, producing distinct traits from each.

Heterozygosity contributes to genetic diversity and can influence fitness. In population genetics, the presence of heterozygotes affects allele frequency dynamics and deviations from simple models. Some heterozygous combinations confer a selective advantage, a phenomenon known as heterozygote advantage or overdominance; a classic example is the sickle cell trait, where heterozygotes have resistance to certain malaria parasites in specific environments.

Overall, heterozygosity is a fundamental concept in genetics, breeding, and evolution. It describes zygosity at a locus, clarifies the potential range of phenotypic outcomes, and underscores the role of genetic diversity in adaptation and inheritance.