The frequency of alleles, genotypes and phenotypes in a population can be calculated using the Hardy–Weinberg equation: p² + 2pq + q² = 1, where p is the frequency of one allele and q is the frequency of the other
The Hardy–Weinberg equation lets you calculate how common each allele, genotype, and phenotype is in a population. You use two values — p and q — which represent the frequencies of the two alleles of a gene.
Formula
p² + 2pq + q² = 1 (where p + q = 1)
Real World
PKU (phenylketonuria) affects about 1 in 10,000 UK births; using q² = 0.0001 gives q = 0.01 and 2pq ≈ 0.02, meaning roughly 1 in 50 people unknowingly carry the recessive allele.
Exam Focus
Show every algebraic step — write q² value, square-root for q, subtract from 1 for p, then calculate 2pq — as method marks outnumber answer marks.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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