The Hardy-Weinberg equation predicts genotype frequencies from allele frequencies when a population is not evolving in evolving populations in evolving populations.
Coverage 2012–2025 · Updated 16 Jul 2026
For two alleles with frequencies p and q, the Hardy–Weinberg model predicts p², 2pq and q² genotype frequencies under specified assumptions.
The equation p+q=1 describes allele frequencies; p²+2pq+q²=1 follows random union of gametes. It provides a null expectation for detecting evolutionary forces.
State p and q; calculate expected genotypes; compare observed values with the model.; separate variation, selection, inheritance and time
If q=0.2, expected aa frequency is q²=0.04 and heterozygote frequency is 2pq=0.32. This gives a concrete prediction from the stated population.
The model is an expectation, not a claim that real populations never evolve. Interpret the result within the stated selection model and evidence limits.
This objective is assessed through essay response, commonly using Outline / Discuss.
Outline / Discuss
Build the answer around this relationship: p and q are allele frequencies that add to one.
Representative question
Discuss the use of the Hardy-Weinberg equation in population genetics studies.
HL D4.1 turns selection into measurable population genetics. A gene pool changes when allele frequencies shift. Hardy-Weinberg gives a no-evolution baseline; selection graphs, isolated populations, artificial selection, and resistance show how forces move populations away from that baseline.
HL population-genetics questions ask students to quantify or model evolution. The answer starts with the gene pool and allele frequencies, then uses the model or selection graph to decide whether the population is at equilibrium or being shifted by selection, mutation, migration, drift, artificial selection, or isolation.
