What is Hardy-Weinberg Equilibrium?
Hardy-Weinberg equilibrium is a fundamental population genetics model that predicts how allele and genotype frequencies stay constant across generations when no evolutionary forces act. It gives biologists a mathematical baseline for detecting when evolution is actually happening in a population.
Hardy-Weinberg equilibrium states that allele frequencies (p, q) and genotype frequencies (p², 2pq, q²) remain constant generation after generation in a large, randomly mating population free from mutation, migration, and selection.
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Step-by-step worked examples
In a population, the frequency of the recessive allele is q = 0.3. Find all genotype frequencies.
p = 1 − q = 1 − 0.3 = 0.7 p² = 0.7² = 0.49 (AA) 2pq = 2 × 0.7 × 0.3 = 0.42 (Aa) q² = 0.3² = 0.09 (aa)
16% of a population shows the recessive phenotype (aa). Find the allele frequencies.
q² = 0.16 → q = √0.16 = 0.4 p = 1 − 0.4 = 0.6 2pq = 2 × 0.6 × 0.4 = 0.48 (carrier frequency)
In a population of 1,000 people, 90 are homozygous recessive (aa). Find the number of heterozygous carriers.
q² = 90/1000 = 0.09 → q = 0.3 p = 1 − 0.3 = 0.7 2pq = 2 × 0.7 × 0.3 = 0.42 Heterozygous individuals = 0.42 × 1000 = 420 people
Flashcards
Quick quiz
Q1.What does p² represent in the Hardy-Weinberg equation?
Q2.If the frequency of the recessive allele is q = 0.2, what is p?
Q3.Which of these is NOT a condition required for Hardy-Weinberg equilibrium?
Q4.A population has q² = 0.25 (frequency of aa individuals). What is q?
The full card deck, worked steps and AI-tutor support for “What is Hardy-Weinberg Equilibrium?” are in Notek — study by hand before your exam.
Common mistakes
Thinking p and q are genotype frequencies. — Correct: p and q are ALLELE frequencies; p², 2pq and q² are the genotype frequencies.
Forgetting that p + q must equal 1. — Correct: Always check p + q = 1 before computing genotype frequencies.
Assuming any real population is in Hardy-Weinberg equilibrium. — Correct: HWE is an idealized baseline — real populations almost always violate at least one assumption.
Believing any deviation from expected frequencies always means natural selection. — Correct: Mutation, migration, genetic drift, or non-random mating can also cause a deviation, not just selection.
FAQ
What is Hardy-Weinberg equilibrium?
It's a population genetics principle stating that allele and genotype frequencies stay constant across generations if no evolutionary forces act on the population.
What is the Hardy-Weinberg equilibrium formula?
p² + 2pq + q² = 1, where p and q are the frequencies of the dominant and recessive alleles (p + q = 1).
How do you calculate Hardy-Weinberg equilibrium?
Find q from the recessive phenotype frequency (q² ), then p = 1 − q, and compute p², 2pq, and q² for the genotype frequencies.
What are some Hardy-Weinberg equilibrium examples?
Calculating carrier frequency of a recessive disease allele, or predicting genotype ratios from a known phenotype ratio — see the worked examples above.