International A Level Biology / Biodiversity and conservation
The Hardy-Weinberg equation revision notes
Edexcel International A Level Biology (YBI11), chapter 12: Biodiversity and conservation, specification point 4.20. Free lesson.
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The Hardy-Weinberg equations link allele frequencies to genotype frequencies in a population. They can help you estimate genotypes from a recessive phenotype and identify when allele frequencies change between generations.
What p, q and the equations mean
The equations apply to a gene with two alleles, one dominant and one recessive. A frequency is a proportion from 0 to 1. Because the two allele frequencies make up the whole population, p + q = 1. p is the frequency of the dominant allele, and q is the frequency of the recessive allele. p² is the frequency of homozygous dominant individuals. 2pq is the frequency of heterozygotes. q² is the frequency of homozygous recessive individuals. The genotype frequencies add to 1: p² + 2pq + q² = 1. Only the homozygous recessive genotype shows the recessive phenotype. So you can…
How to find heterozygotes from a recessive phenotype
Start with the recessive phenotype because it gives you q². Find q by taking the square root, then use p + q = 1 to find p. Calculate 2pq for the heterozygote frequency. If the question asks for a number of individuals, multiply that frequency by the population total.
What keeps a population in Hardy-Weinberg equilibrium
Hardy-Weinberg equilibrium means allele frequencies stay the same each generation. This is expected only when all five conditions hold. If a condition is broken, allele or genotype frequencies can change. The population is very large. In a small population, chance alone can shift allele frequencies. Mating is random. If individuals choose mates by genotype, genotype frequencies stop matching the equation. There is no mutation, so no new alleles appear. There is no natural selection. Otherwise, alleles that help individuals survive and reproduce may be passed on more. There…
How to check for a change between generations
One calculation gives allele frequencies for one generation, but it cannot show whether they have changed. Use the same method in two generations from the same population, then compare the results. If q is different, the allele frequency is changing and the population is not in equilibrium.