Heterozygosity Calculators

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Heterozygosity is the proportion of individuals in a population carrying two different alleles at a given genetic locus. It is a fundamental measure of genetic diversity. Observed heterozygosity (Ho) is directly measured from genotype data. Expected heterozygosity (He) is predicted from allele frequencies under Hardy-Weinberg equilibrium: He = 1 − Σpᵢ². Comparing Ho and He reveals inbreeding, population substructure, or natural selection. Low heterozygosity in small populations is associated with inbreeding depression and reduced adaptive capacity.

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Observed Heterozygosity (Ho)

Ho = number of heterozygous individuals / total individuals sampled

Count individuals carrying two different alleles at the locus and divide by total genotyped. If 35 out of 100 are heterozygotes: Ho = 0.35.

Expected Heterozygosity (He)

Under Hardy-Weinberg equilibrium with allele frequencies p₁, p₂,..., pₙ:

He = 1 − Σpᵢ²

For a biallelic locus: He = 2pq (where p + q = 1). He is maximized at 0.5 when p = q = 0.5. He is also called gene diversity — the probability that two randomly chosen alleles differ.

Inbreeding Coefficient Fis

Fis = 1 − (Ho / He). Fis > 0 indicates heterozygote deficit (inbreeding or Wahlund effect). Fis < 0 indicates excess heterozygosity (outbreeding). Fis = 0 indicates Hardy-Weinberg equilibrium.

Conservation Genetics

Small populations lose heterozygosity through genetic drift at approximately 1/(2Ne) per generation, where Ne is effective population size. Low He is associated with inbreeding depression, impaired immunity, and reduced adaptive potential. Translocation between isolated populations (genetic rescue) raises heterozygosity and can dramatically improve fitness.

Glossary

Observed Heterozygosity (Ho)
The proportion of heterozygous individuals at a locus measured directly from genotype data; compared to He to detect deviations from Hardy-Weinberg equilibrium.
Expected Heterozygosity (He)
Predicted proportion of heterozygotes under HWE: He = 1 − Σpᵢ²; also called gene diversity; the probability that two random alleles from the population differ.
Inbreeding Coefficient (Fis)
Fis = 1 − (Ho/He); positive values indicate inbreeding or population substructure; negative values indicate outbreeding; zero indicates Hardy-Weinberg equilibrium.

Frequently Asked Questions

Observed heterozygosity (Ho) is directly counted from genotype data: heterozygotes / total sampled. Expected heterozygosity (He) is calculated from allele frequencies assuming Hardy-Weinberg equilibrium: He = 1 − Σpᵢ². If Ho ≈ He, the locus is in HWE. If Ho < He, there is a heterozygote deficit indicating inbreeding or population substructure. If Ho > He, there is excess heterozygosity indicating outbreeding or balancing selection.

For a biallelic locus with allele frequencies p and q (p + q = 1): He = 2pq. For multiple alleles: He = 1 − Σpᵢ², where pᵢ are individual allele frequencies. He is maximized when all alleles are at equal frequency. It represents the probability that two randomly drawn alleles from the population are different — also called gene diversity.

Heterozygosity measures genetic diversity within a population. Low He signals loss of genetic variation through drift, inbreeding, or bottlenecks. Inbreeding depression — reduced fitness in inbred individuals — is driven by homozygosity exposing deleterious recessive alleles. Conservation programs track He over time, and when it drops critically low, genetic rescue (translocation of individuals from other populations) is used to restore diversity and fitness.

Fis = 1 − (Ho/He). It quantifies the deviation of observed heterozygosity from Hardy-Weinberg expectations: Fis = 0 means HWE; Fis > 0 means heterozygote deficit (inbreeding or Wahlund effect from population subdivision); Fis < 0 means heterozygote excess (outbreeding or heterozygote advantage). At the individual level, F is the probability that both alleles are identical by descent from a common ancestor.