Explain allele frequencies and gene pools, and use the Hardy-Weinberg principle to detect change

What this dot point is asking

You need to define gene pool and allele frequency, state the Hardy-Weinberg conditions, and use the Hardy-Weinberg equations to calculate allele and genotype frequencies. This dot point treats evolution quantitatively.

Gene pools and allele frequencies

A gene pool is the total collection of all alleles for all genes in a population. The allele frequency is the proportion of a particular allele among all the copies of that gene in the population.

Evolution can be defined at this level as a change in allele frequencies in a gene pool over generations. The mechanisms that change them are natural selection, genetic drift, gene flow and mutation.

The Hardy-Weinberg principle

The Hardy-Weinberg principle describes a theoretical population in which allele frequencies stay constant from generation to generation - a population that is not evolving. It acts as a null model: if a real population matches it, no evolution is occurring; if it deviates, evolution is.

It uses two equations, where $p$ and $q$ are the frequencies of two alleles of a gene:

Here $p^2$ is the frequency of the homozygous dominant genotype, $2pq$ the heterozygous genotype, and $q^2$ the homozygous recessive genotype.

The conditions for equilibrium

Allele frequencies stay constant only if all the following hold:

• a large population (so drift is negligible)
• no mutation
• no gene flow (no migration)
• random mating
• no natural selection

If any condition is broken, allele frequencies can change - that is, evolution can occur.