Given the frequency of a dominant allele in a population, this finds the expected genotype frequencies under Hardy-Weinberg equilibrium.
How it works
The recessive allele frequency is found by subtracting from 1, then the genotype frequencies are computed as p², 2pq, and q².
What this does not include
This does not include a single genetic cross between two known individuals — for that, use this site’s Punnett square calculator instead. This model also assumes an idealized population with no mutation, migration, selection, or genetic drift.
How to use this calculator
- Enter the dominant allele frequency (p) as a decimal between 0 and 1.
A worked example
Dominant allele frequency p = 0.6: recessive frequency q = 1 − 0.6 = 0.4. Genotype frequencies: AA = p² = 36%, Aa = 2pq = 48%, aa = q² = 16%.
p = 0.9: q = 0.1, giving AA = 81%, Aa = 18%, aa = 1%.
What the variables mean
| Variable | Meaning |
|---|---|
| p | Frequency of the dominant allele in the population |
| q (= 1 − p) | Frequency of the recessive allele |
| p², 2pq, q² | Expected homozygous dominant, heterozygous, and homozygous recessive frequencies |
Edge cases worth knowing
2pq (not pq) is used for heterozygotes because there are two ways to be heterozygous — inheriting the dominant allele from either parent — so the two equal-probability paths are added together.
This model assumes an idealized population — no mutation, migration, selection, or genetic drift — a theoretical baseline used to detect when a real population is actually evolving away from it.
Frequently asked questions
What does Hardy-Weinberg equilibrium actually describe?
A theoretical population where allele and genotype frequencies stay constant across generations, used as a baseline to detect when real populations are evolving.
Why does the heterozygous frequency use 2pq instead of pq?
Because there are two ways to be heterozygous — inheriting the dominant allele from either parent — so the two equal probabilities (pq and qp) are added together.
What happens if p is 1?
Every individual carries only the dominant allele, so the population is 100% homozygous dominant with no recessive allele present at all.
