🎓 Prepared by students from Boğaziçi University

What is Population Genetics?

Population genetics studies how allele and genotype frequencies change (or stay stable) in a population over time. The Hardy-Weinberg principle is its foundational null model, predicting genotype frequencies when a population is NOT evolving.

Short answer

Population genetics is the study of allele and genotype frequency distributions within populations; the Hardy-Weinberg equation p² + 2pq + q² = 1 predicts these frequencies when no evolutionary forces are acting.

Hardy-Weinberg Genotype Frequencies vs. Allele Frequency p

x: p (dominant allele frequency) · y: Genotype frequencyp² (homozygous dominant)2pq (heterozygous)q² (homozygous recessive)

Try it: interactive calculator

Heterozygote frequency (2pq)

0.5

= 2*0.5*(1-0.5)

Dominant allele frequency (p)0.5

Step-by-step worked examples

In a population, the recessive allele frequency is q = 0.3. Find p and the genotype frequencies p², 2pq, and q².

p = 1 − q = 1 − 0.3 = 0.7
p² = 0.7² = 0.49
2pq = 2 × 0.7 × 0.3 = 0.42
q² = 0.3² = 0.09

9% of a population shows a recessive phenotype (q² = 0.09). Find the allele frequencies p and q.

q = √0.09 = 0.3
p = 1 − q = 1 − 0.3 = 0.7

In a population of 1000 people with p = 0.6, how many are expected to be heterozygous carriers?

2pq = 2 × 0.6 × 0.4 = 0.48
Expected carriers = 0.48 × 1000 = 480 people

Flashcards

Quick quiz

Q1.What is the Hardy-Weinberg equation?

Correct answer: A. This equation describes genotype frequencies in a non-evolving population.

Q2.If q = 0.4, what is p?

Correct answer: B. Since p + q = 1, p = 1 − 0.4 = 0.6.

Q3.Which term in the Hardy-Weinberg equation represents heterozygotes?

Correct answer: C. 2pq is the frequency of heterozygous individuals.

Q4.Which of these is NOT a Hardy-Weinberg assumption?

Correct answer: C. Hardy-Weinberg assumes NO genetic drift, which requires an infinitely large population.

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Common mistakes

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p and q represent genotype frequencies. — Correct: p and q represent allele frequencies; p², 2pq, and q² are the genotype frequencies.

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Hardy-Weinberg describes a population that is actively evolving. — Correct: It describes a population NOT evolving — it's a null/equilibrium model.

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2pq represents homozygous individuals. — Correct: 2pq represents heterozygous individuals; p² and q² are the homozygotes.

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Small populations always satisfy Hardy-Weinberg equilibrium. — Correct: Hardy-Weinberg requires a very large population to avoid genetic drift disrupting the equilibrium.

FAQ

What is population genetics?

Population genetics is the study of how allele and genotype frequencies are distributed and change within populations over time.

What is the Hardy-Weinberg formula?

p² + 2pq + q² = 1, where p and q are the frequencies of the dominant and recessive alleles (p + q = 1).

What are examples of population genetics calculations?

Given q = 0.3, you can find p = 0.7 and genotype frequencies p² = 0.49, 2pq = 0.42, q² = 0.09.

How do you calculate allele frequency in population genetics?

If you know a recessive phenotype's frequency (q²), take its square root to get q, then p = 1 − q.