Population genetics: Hardy–Weinberg and carrier frequency
How common a disease-causing allele is in a population, and why it can be more frequent in some groups than in others.
Key facts
- Hardy–Weinberg: with two alleles of frequencies p and q (p + q = 1), a large, randomly mating population has genotypes in the proportions p² (AA), 2pq (Aa) and q² (aa).
- For a rare autosomal recessive disease, q² is the disease frequency, so q ≈ √(prevalence) and carriers are about 2q.
- Example: if 1 in 10 000 people is affected, q = 1/100 and about 1 in 50 people is a carrier.
- For an X-linked recessive trait, the frequency in males equals the allele frequency q; affected females are much rarer (q²).
- The equilibrium assumes no selection, mutation or migration, random mating and a large population; real populations depart from it.
In detail
- Founder effect: a small group that starts a new population carries, by chance, its own mix of alleles; a rare variant can become common among its descendants.
- Genetic drift: random changes in allele frequency from one generation to the next, strongest in small or isolated populations.
- Consanguinity (parents related by descent) raises the chance that a child receives the same rare recessive allele from both sides.
- Heterozygote advantage keeps some harmful alleles common: carriers of HbS are partly protected against severe falciparum malaria.
- Allele frequencies differ between populations, so carrier screening and the reading of genetic tests depend on ancestry.
- Also called
- Hardy–Weinberg equilibrium, Founder effect
Clinical notes
- Carrier-risk calculations in genetic counselling start from the population carrier frequency.
- Where consanguinity is common or a population descends from few founders, rare recessive diseases appear more often than expected.
- Two carriers of the same recessive allele have a 1 in 4 chance of an affected child in each pregnancy.
Test yourself
Think of the answer, then tap the question to see it.
A recessive disease affects 1 in 2 500 people. About how many people are carriers?
q² = 1/2 500, so q = 1/50; carriers ≈ 2q = 1/25, about 1 person in 25.
Why is the sickle cell allele common where malaria is frequent?
Carriers (HbAS) are partly protected against severe falciparum malaria, so the allele stays in the population: heterozygote advantage.
What is a founder effect?
A rare allele becomes common because the small group that founded a population happened to carry it.
Connections
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Why are some genetic conditions more common in particular ethnic groups?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Founder Effect (National Human Genome Research Institute (NHGRI), 2026); Genetic Drift (National Human Genome Research Institute (NHGRI), 2026).
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Representative model, for educational purposes only.