Heterozygote Calculators

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A heterozygote is an individual carrying two different alleles at a given genetic locus — one on each homologous chromosome (e.g., Aa for a diploid organism). Heterozygosity is a key measure of genetic variation in populations: observed heterozygosity (Ho = proportion of heterozygous individuals) and expected heterozygosity under Hardy-Weinberg equilibrium (He = 2pq for a biallelic locus; He = 1 − Σpᵢ² for multi-allelic loci). Heterozygote advantage (overdominance) — where the heterozygote has higher fitness than either homozygote — maintains genetic polymorphisms through balancing selection. Classic example: HbA/HbS heterozygotes have higher fitness in malaria-endemic regions.

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Heterozygosity Measures

For a diploid population of N individuals. Observed heterozygosity: Ho = number of Aa individuals / N. Expected heterozygosity (HWE): He = 1 − Σpᵢ² (for multi-allelic locus). For biallelic: He = 2pq. He also equals mean probability that two alleles drawn from the population are different. Ho < He → inbreeding (FIS > 0). Ho > He → balancing selection or recent admixture.

Inbreeding Coefficient

FIS = 1 − Ho/He. FIS = 0: observed heterozygosity matches HWE expectation. FIS > 0: deficit of heterozygotes (inbreeding, population structure). FIS < 0: excess heterozygotes (outbreeding, balancing selection).

Heterozygote Advantage

Fitness: w_AA < w_Aa > w_aa → overdominance → both alleles maintained. Examples: HbA/HbS sickle cell heterozygotes: partially resistant to severe malaria; higher fitness in malaria-endemic regions than HbAA. MHC (major histocompatibility complex): broader pathogen recognition → heterozygote advantage → extremely high MHC polymorphism maintained. ABO blood group: some heterozygote advantage hypothesized.

Glossary

Heterozygote
An individual carrying two different alleles at a locus (Aa); Ho = observed heterozygosity = fraction of heterozygous individuals; He = 1−Σpᵢ² = expected heterozygosity under HWE.
FIS (Inbreeding Coefficient)
FIS = 1 − Ho/He; measures excess or deficit of heterozygotes; FIS > 0 = excess homozygotes (inbreeding); FIS < 0 = excess heterozygotes (outbreeding or balancing selection); ranges −1 to +1.
Heterozygote Advantage
Fitness of Aa > AA and aa (overdominance); maintains both alleles by balancing selection; classic example: HbA/HbS in malaria-endemic regions; also important at MHC loci.

Frequently Asked Questions

A heterozygote carries two different alleles at a locus (e.g., Aa, A₁A₂). Heterozygosity measures: Observed heterozygosity (Ho): the proportion of individuals in a sample that are heterozygous = n_heterozygotes / N_total. Expected heterozygosity (He) under HWE: He = 1 − Σpᵢ² for any number of alleles; = 2pq for a biallelic locus. He also = the probability that two randomly sampled alleles from the population are different (genetic diversity). Comparison: Ho ≈ He: population consistent with random mating. Ho < He: excess homozygotes → inbreeding, population structure. Ho > He: excess heterozygotes → balancing selection, recent admixture, or recent bottleneck.

Heterozygote advantage (overdominance): the fitness of the heterozygote (Aa) exceeds the fitness of either homozygote (AA or aa). Consequence: neither allele can be eliminated by natural selection — both are maintained in the population (balancing selection). Allele frequencies reach a stable equilibrium: p̂ = (s_aa) / (s_AA + s_aa), where s = selection coefficients. Classic example: sickle cell (HbA/HbS): HbSS: sickle cell disease → severe anemia, early death → low fitness. HbAA: susceptible to severe falciparum malaria → reduced fitness in endemic areas. HbAS: partial resistance to malaria, no sickle cell disease → highest fitness. Result: HbS frequency maintained at ~10–40% in malaria-endemic West Africa.

Heterozygosity indicates the genetic diversity of a population: High He: more genetic variation → more evolutionary potential; better ability to adapt to new environments or pathogens. Low He: inbred or bottlenecked population; reduced fitness (inbreeding depression); less adaptive potential. Inbreeding coefficient FIS = 1 − Ho/He: FIS > 0: more inbreeding than expected → concern for small or isolated populations. Conservation thresholds: minimum viable population (Ne ≥ 50) → prevents rapid loss of heterozygosity (ΔHe < 1%/generation). Ne ≥ 500 → maintains evolutionary potential. Genetic rescue: introducing migrants from outside populations → increases Ho → reduces FIS → improves fitness. Monitoring: track He over time; declining He = ongoing genetic erosion in isolated populations.

FIS (inbreeding coefficient within subpopulations) = 1 − Ho/He. F = 0: observed heterozygosity matches HWE expectation → random mating. F > 0: deficit of heterozygotes; caused by: inbreeding (mating between relatives); population structure (Wahlund effect — mixing subpopulations with different allele frequencies looks like inbreeding); assortative mating; genotyping errors (heterozygote dropout). F < 0: excess heterozygotes; caused by: outbreeding (avoidance of mating with relatives — common in many species); balancing selection maintaining heterozygotes. F = 0.125: equivalent to mating with a half-sibling. F = 0.25: equivalent to mating with a full sibling. Conservation: F > 0.10 is often a threshold for concern; indicates substantial inbreeding.