Heterozygosity Calculator
Observed vs. Expected: A Core Population Genetics Comparison
Heterozygosity — the proportion of individuals carrying two different alleles at a locus — can be measured two ways: counted directly from a sample (observed), or predicted from the allele frequency under Hardy-Weinberg assumptions (expected). Comparing the two is one of the most common diagnostic checks in population genetics, since a meaningful gap between them signals something biologically interesting is happening at that locus.
The Formula
Expected Heterozygosity (He) = 2pq
He follows directly from the Hardy-Weinberg genotype proportions (p² + 2pq + q² = 1): the middle term, 2pq, is the expected proportion of heterozygotes if the population were mating randomly with no other evolutionary forces at play.
Worked Example
A sample of 100 individuals with 40 heterozygotes, and an A allele frequency of p = 0.6:
| Measure | Value |
|---|---|
| Observed Heterozygosity (Ho) | 0.4000 |
| Expected Heterozygosity (He) | 0.4800 |
Interpreting the Gap Between Ho and He
- Ho notably lower than He — suggests inbreeding, population subdivision, or non-random mating, all of which reduce the actual rate of heterozygous offspring below the random-mating expectation.
- Ho close to He — consistent with a randomly mating population near Hardy-Weinberg equilibrium at that locus.
- Ho notably higher than He — can indicate heterozygote advantage (overdominance) or outbreeding between genetically distinct subpopulations.
Where This Calculation Matters
- Conservation genetics — low observed heterozygosity relative to expectation is a standard early warning sign of a shrinking, inbreeding population.
- Marker validation in genotyping studies — a large Ho/He gap at a specific marker can flag technical genotyping errors rather than real biology.
- Evolutionary biology research — heterozygosity comparisons across loci help detect regions of the genome under selection.
How to Use This Calculator
- Enter the number of heterozygous individuals in the sample.
- Enter the total number of individuals sampled.
- Enter the allele frequency p (between 0 and 1).
- Select Calculate to get both observed and expected heterozygosity.
Related Calculations
Test the same population directly against equilibrium expectations with the Hardy-Weinberg Calculator, or derive p from raw genotype counts with the Allele Frequency Calculator.