A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed. The individuals in a population of a species may show a wide range of variation in phenotype. This is the result of genetic and environmental factors. A gene pool is all the alleles of all the genes in a population. The frequency of an allele in a population is the proportion of organisms carrying that allele. The Hardy-Weinberg principle provides a mathematical model, which predicts that allele frequencies will not change from one generation to the next, given that no mutation, migration, selection or genetic drift occurs and that there is random mating in a large population. Since allele frequencies do change, the conditions required to maintain a Hardy-Weinberg equilibrium are rarely met. Students should be able to use the Hardy-Weinberg principle (p + q = 1 and p2 + 2pq + q2 = 1) to calculate allele, genotype and phenotype frequencies in populations and changes in these frequencies.

What this dot point is asking

AQA wants you to define a population, gene pool and allele frequency, state the conditions for Hardy-Weinberg equilibrium, and use the two Hardy-Weinberg equations to calculate allele, genotype and phenotype frequencies (and changes in them) from data.

Phenotypic variation within a population results from both genetic factors (different alleles, meiosis, random fertilisation, mutation) and environmental factors (diet, climate, lifestyle), which usually act together.

The Hardy-Weinberg principle

The Hardy-Weinberg principle is a mathematical model predicting that allele frequencies stay constant from generation to generation, provided a set of conditions holds. It gives a baseline: if real allele frequencies do change, one of the conditions has been broken, which signals evolution.

Conditions for equilibrium

Hardy-Weinberg equilibrium holds only when all five conditions are met:

1. No mutation (no new alleles are introduced).
2. No migration (no immigration or emigration changing the gene pool).
3. No natural selection (all genotypes are equally fertile and viable).
4. No genetic drift (the population is very large, so chance has negligible effect).
5. Random mating (mating is not influenced by genotype).

Because these conditions are rarely all met in real populations, true equilibrium is uncommon, and observed frequency change is evidence of evolutionary processes at work.

Working out allele frequencies

The recessive phenotype is the only one whose genotype is certain from the phenotype: it must be homozygous recessive, frequency $q^2$. So always start by finding $q^2$ from the recessive phenotype, take its square root to find $q$, then use $p = 1 - q$.

Detecting changes in frequency

To show a population is evolving, compare allele frequencies over time. If $q$ rises or falls between generations, equilibrium has been broken, usually by selection (for example, an antibiotic-resistance allele rising under antibiotic use) or by genetic drift in a small population.

Try this

Q1. State the five conditions that must be met for a population to be in Hardy-Weinberg equilibrium. [3 marks]

• Cue. No mutation, no migration, no natural selection, no genetic drift (large population), and random mating.

Q2. A recessive allele causes a condition affecting 1 in 10 000 people. Calculate (a) the frequency of the recessive allele and (b) the percentage of the population that are carriers. [3 marks]

• Cue. (a) $q^2 = 0.0001$, so $q = 0.01$. (b) $p = 0.99$; carriers $2pq = 2 \times 0.99 \times 0.01 = 0.0198 = 1.98$ percent.

Q3. A biologist samples a population in two successive generations and finds the recessive allele frequency rises from 0.2 to 0.35. Explain what this shows and name one process that could cause it. [3 marks]

• Cue. Allele frequency has changed, so the population is not in Hardy-Weinberg equilibrium and is evolving; natural selection favouring the recessive allele (or genetic drift, or migration) could cause it.