Population Genetics and Hardy-Weinberg Equilibrium

Core Idea

Population genetics studies how allele and genotype frequencies change across generations in a population. The Hardy-Weinberg principle states that, under idealized conditions (large population, random mating, no mutation, migration, or selection), allele frequencies remain constant from generation to generation, and genotype frequencies satisfy p² + 2pq + q² = 1. Deviations from Hardy-Weinberg equilibrium signal one or more evolutionary forces acting on the population. Natural selection, genetic drift, mutation, gene flow, and non-random mating all alter allele frequencies, driving evolutionary change.

How It's Best Learned

Calculate expected Hardy-Weinberg genotype frequencies from observed allele frequencies and test for deviations using chi-square analysis. Practice identifying which violation of assumptions would cause each type of deviation.

Common Misconceptions

• Hardy-Weinberg is a null model, not a description of real populations; it is useful precisely because real populations deviate from it.
• p + q = 1 describes allele frequencies, not genotype frequencies; p² + 2pq + q² = 1 describes genotype frequencies.

Explainer

From Mendelian genetics, you know how traits are inherited in single crosses: dominant and recessive alleles segregate according to predictable ratios. Population genetics asks a different question: across an entire population breeding over many generations, how do allele frequencies change — or stay the same?

The Hardy-Weinberg principle answers the "stay the same" case. Under five idealized conditions — infinite population size, random mating, no mutation, no migration, and no natural selection — allele frequencies remain constant indefinitely. If allele A has frequency p and allele a has frequency q (with p + q = 1), then genotype frequencies in the next generation will be p² (AA), 2pq (Aa), and q² (aa). This is simply the result of random mating: each offspring independently draws one allele from each parent at random, so the probability of AA is p × p = p². The 2pq term for heterozygotes gets the factor of 2 because there are two ways to combine the alleles (A from mom and a from dad, or vice versa).

The real power of Hardy-Weinberg is not as a description of real populations — those five conditions are never all met simultaneously — but as a null model. It tells you what to expect if *nothing* is happening evolutionarily. When you observe a population and its genotype frequencies deviate from p² + 2pq + q², that deviation is a signal. An excess of homozygotes suggests inbreeding or assortative mating. A shift in allele frequencies over generations suggests selection, drift, or gene flow. Hardy-Weinberg equilibrium is the baseline; evolution is the deviation from it.

Keep careful track of what p and q describe: they are allele frequencies, not genotype frequencies. In a population where 36% of individuals are homozygous recessive (aa), q² = 0.36, so q = 0.6 and p = 0.4. From those allele frequencies you can calculate all three expected genotype frequencies. This inferential direction — from observable genotype counts back to allele frequencies, then forward to predictions — is the core workflow of population genetics analysis.