Heterozygosity

Heterozygosity, in genetics, refers to the presence of different alleles at a gene locus within an individual. In diploid organisms, a heterozygous genotype contains two different alleles, such as A and a. Heterozygosity within a population is a measure of genetic variation and can be assessed at many loci across the genome.

Observed heterozygosity Ho is defined as the proportion of individuals in a sample that are heterozygous at a given locus. Expected heterozygosity He, derived under Hardy-Weinberg equilibrium, equals 1 minus the sum of the squared allele frequencies: He = 1 − Σ p_i^2 (for multiple alleles); for a two-allele locus, He = 2pq.

Comparing Ho and He can reveal deviations from random mating. If Ho < He, inbreeding or population subdivision may be present; if Ho > He, balancing selection or assortative mating might occur. The inbreeding coefficient F_IS can be estimated as 1 − Ho/He.

Applications and significance: Heterozygosity is a key measure of genetic diversity within populations and is used in conservation genetics, ecology, and evolutionary biology to track genetic health, assess effects of drift, migration, and selection, and monitor changes over time. Genome-wide or SNP-based heterozygosity provides a broad view of diversity but can be influenced by sampling and ascertainment bias.