# Hardy-Weinberg equilibrium

A model for a population that is not evolving. Allele frequencies stay constant if there is no selection, mutation, drift or gene flow, and mating is random. If p and q are the two allele frequencies, p + q = 1 and genotype frequencies are p^2 + 2pq + q^2 = 1.

Biology (university) · Part 08: Evolution and diversity of life · Card 5 of 8 · Last updated 2026-10-19

## Analogy

A control experiment: what would happen if nothing changed?

## Example

**Carrier count**

1 in 2,500 have disease.
q^2 = 0.0004, so q = 0.02.
p = 0.98.
2pq = 0.039.
About 1 in 25 carry it.

## In practice

It lets us estimate how many carriers of a recessive disease are in a population.

## Practice

### Flashcards

- **State the Hardy-Weinberg equations.** p + q = 1 and p^2 + 2pq + q^2 = 1.

### Interview questions

**List the conditions for Hardy-Weinberg equilibrium and explain why real populations rarely meet them.**

- No mutation
- No selection
- No gene flow
- Large population, no drift
- Random mating

## Questions people ask

### Why is Hardy-Weinberg useful if populations never meet it?

It gives a baseline. If real allele frequencies differ from the predictions, something is acting on the population, such as selection, drift or non-random mating. It is also used to estimate carrier numbers for recessive diseases from the frequency of affected individuals.

### How do I use the Hardy-Weinberg equation?

Start from what you can count, usually the frequency of the recessive phenotype, which equals q squared. Take the square root to get q, then calculate p as 1 minus q. Heterozygotes are 2pq, and the dominant homozygotes are p squared.

## Related concepts

- [Genetic drift](https://studyvisually.com/learn/science/biology-university/biology-university-part-8/biology-university-part-8-concept-3)
- [Gene flow](https://studyvisually.com/learn/science/biology-university/biology-university-part-8/biology-university-part-8-concept-4)
- [Natural selection](https://studyvisually.com/learn/science/biology-university/biology-university-part-8/biology-university-part-8-concept-1)
- [Fitness](https://studyvisually.com/learn/science/biology-university/biology-university-part-8/biology-university-part-8-concept-2)

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