Gene Diversity Calculators

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Gene diversity — formally known as expected heterozygosity (He) — is the most widely used measure of genetic variation within a population. It represents the probability that two randomly drawn alleles from a population are different, calculated from allele frequency data. Gene diversity is a key parameter in population genetics, conservation biology, and breeding programs — it quantifies the genetic richness of a population and predicts its capacity to adapt. Low gene diversity is a warning sign of inbreeding, bottleneck effects, and reduced evolutionary potential.

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What Is Gene Diversity?

Gene diversity, also called expected heterozygosity (He) or Nei's gene diversity, is defined as the probability that two alleles randomly drawn from a population are different from one another:

He = 1 − Σpᵢ²

Where pᵢ is the frequency of the i-th allele at a locus. Summing the squared frequencies of all alleles gives the probability of drawing two identical alleles (homozygosity); subtracting from 1 gives heterozygosity.

He ranges from 0 (only one allele — monomorphic locus) to approaching 1 (many alleles at equal frequencies). For a locus with 2 alleles at equal frequency (p = q = 0.5): He = 1 − (0.25 + 0.25) = 0.5.

Gene Diversity vs. Observed Heterozygosity

  • Expected heterozygosity (He): Calculated from allele frequencies using Hardy-Weinberg assumptions. Represents what heterozygosity should be if the population is random-mating.
  • Observed heterozygosity (Ho): The actual proportion of individuals that are heterozygous at a locus, determined by genotyping.

Comparing Ho and He reveals population structure:

  • Ho ≈ He: Random mating, Hardy-Weinberg equilibrium
  • Ho < He: Inbreeding, Wahlund effect (population substructure), or selection against heterozygotes
  • Ho > He: Heterozygote advantage (overdominance) or negative assortative mating

Average Gene Diversity Across Loci

In practice, gene diversity is averaged across multiple loci:

H̄e = (1/L) Σ Heᵢ

Where L is the number of loci and Heᵢ is the gene diversity at locus i. This multilocus average is more reliable than single-locus estimates for characterizing population-level diversity.

Gene Diversity in Conservation

Conservation biologists monitor gene diversity to assess population health. Low He (<0.1–0.2) indicates reduced diversity and is associated with:

  • Inbreeding depression: Reduced survival, fertility, and disease resistance
  • Limited adaptive potential: Fewer alleles available for selection to act on
  • Increased extinction risk in changing environments

Cheetahs (He ~0.01–0.03) and northern elephant seals (recovered from ~20 individuals; He ~0.03) are classic examples of species with extremely low gene diversity from historical bottlenecks.

Glossary

Gene Diversity (He)
The probability that two randomly drawn alleles from a population differ, calculated as He = 1 − Σpᵢ². Also called expected heterozygosity or Nei's gene diversity. A standard measure of genetic variation within populations.
Observed Heterozygosity (Ho)
The proportion of individuals in a sample that are heterozygous at a given locus, measured directly by genotyping. Compared to expected heterozygosity to assess departures from Hardy-Weinberg equilibrium.
Inbreeding Depression
Reduced fitness (survival, fertility, disease resistance) in offspring of closely related parents, caused by increased homozygosity revealing deleterious recessive alleles. A major concern for small, isolated populations with low gene diversity.

Frequently Asked Questions

Gene diversity (expected heterozygosity, He) measures the probability that two randomly chosen alleles from a population are different. It is calculated from allele frequencies as He = 1 − Σpᵢ², where pᵢ is the frequency of each allele. Values range from 0 (monomorphic — only one allele) to near 1 (many alleles at equal frequencies).

Expected heterozygosity (He) is calculated theoretically from allele frequencies assuming random mating. Observed heterozygosity (Ho) is measured directly by genotyping individuals and counting those that are heterozygous. When Ho < He, it suggests inbreeding or population substructure. When Ho > He, it suggests heterozygote advantage or negative assortative mating.

Low gene diversity in small or isolated populations indicates limited genetic variation for natural selection to act on, increased inbreeding and associated fitness depression, and reduced ability to adapt to environmental change or disease. Captive breeding programs and managed translocations aim to maintain or restore gene diversity to prevent inbreeding depression and preserve adaptive potential.

The Hardy-Weinberg principle predicts genotype frequencies in a large, randomly mating population with no selection, mutation, or migration. Expected heterozygosity (He) is the heterozygosity predicted under Hardy-Weinberg equilibrium from observed allele frequencies. Comparing He to observed heterozygosity (Ho) is a standard way to test whether a population deviates from Hardy-Weinberg expectations.