Hardy-Weinberg equation:
story, analogy and memory trick
The Hardy-Weinberg equation is p² + 2pq + q² = 1. In a population that is not evolving, allele frequencies p and q stay constant, and genotype frequencies follow p² + 2pq + q² = 1.
The Hardy-Weinberg equation
- pfrequency of the dominant allele
- qfrequency of the recessive allele
- p²homozygous dominant individuals
- 2pqheterozygous individuals
- q²homozygous recessive individuals
The story
In a village, every gene comes in two versions, dominant p and recessive q, and marriages are random. Two dominant copies: p × p. Two recessive copies: q × q. One of each can happen in two ways: 2pq. Every villager falls in one of these groups, so together they make 1.
Real-life analogy
It is just (p + q)² expanded. Because p + q = 1, squaring both sides gives p² + 2pq + q² = 1.
Memory trick
Remember the algebra identity (a + b)² = a² + 2ab + b². Replace a with p and b with q.
Exam tip
It assumes no mutation, no migration, no natural selection, random mating and a very large population. If q² is given, take the square root to find q, then p = 1 − q.
Questions about Hardy-Weinberg
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