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Alleles · genotype frequencies · populations
Hardy–Weinberg calculations
Calculate allele frequencies, heterozygote frequencies and expected genotype counts from a stated two-allele Hardy–Weinberg model.
Method and worked examples ↓Learn the method
How to approach these questions
- Identify whether the supplied quantity is an allele frequency, genotype frequency or phenotype frequency.
- Convert a percentage into a decimal before using the equations.
- If the recessive phenotype frequency is supplied, q is its square root; then p = 1 − q.
- Heterozygote frequency is 2pq. Multiply a genotype frequency by population size for an expected count.
- The model assumes random mating, a sufficiently large population and no selection, migration or mutation changing the allele frequencies. It is a model, not proof that a real population meets those assumptions.
Worked examples
Under the two-allele equilibrium model, 9% show the recessive phenotype. Find q, p and the heterozygote frequency.
q² = 0.09, so q = 0.3 and p = 0.7. Heterozygote frequency = 2 × 0.7 × 0.3 = 0.42.
The dominant phenotype frequency is 0.84. Find the dominant allele frequency p under the stated model.
The recessive phenotype frequency is 1 − 0.84 = 0.16. q = √0.16 = 0.4, so p = 0.6. The dominant phenotype frequency itself is not p.
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Topic map and study list · Practical data guides · Active recall decks
Course resources
AQA A-level Biology · OCR A A-level Biology · Textbook answers
Skill coverage follows the common A-level science subject content. Check your board’s current requirements: AQA 7402 and OCR A H420.