---
title: "The Hardy-Weinberg Principle and Allele Frequencies"
description: "In a population that meets a short list of conditions, allele and genotype frequencies stay put from one generation to the next. Departures from that prediction are how selection gets detected."
canonical: https://lightmysky.com/learn/science/the-hardy-weinberg-principle-and-allele-frequencies-mt_eJjwZ2qVAE
source: https://lightmysky.com/learn/science/the-hardy-weinberg-principle-and-allele-frequencies-mt_eJjwZ2qVAE.md
retrieved: 2026-09-12
---

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# The Hardy-Weinberg Principle and Allele Frequencies

In a population that meets a short list of conditions, allele and genotype frequencies stay put from one generation to the next. Departures from that prediction are how selection gets detected.

Subject: Science · Area: Genetics & Evolution · Ages 17 to 18
Page: https://lightmysky.com/learn/science/the-hardy-weinberg-principle-and-allele-frequencies-mt_eJjwZ2qVAE

## Ready when they can

- Calculates allele frequencies from genotype counts and genotype counts from allele frequencies.
- Lists the conditions the model assumes and names a real population that breaks one of them.
- Explains why a change in the frequencies across generations counts as evidence of selection.

## Lesson: When gene pools stand still

Hardy-Weinberg is the calm baseline of evolution. It says that with no disturbing forces, allele and genotype frequencies sit still forever, so the population is not evolving for that gene. Call the two allele frequencies p and q. Then p plus q equals 1, and the genotype mix is p squared plus 2pq plus q squared equals 1.

Count alleles straight from genotype numbers. Tally each homozygote twice and each heterozygote once, then divide by all alleles at that gene. Say 10 AA, 20 Aa and 70 aa live in a group of 100: A copies are 20 plus 20, which is 40 of 200 alleles, so p is 0.2. Run it backwards to predict genotype counts from p and q, and to find carriers: with 1 in 100 affected, q squared is 0.01, q is 0.1, and carriers are 2 times 0.9 times 0.1, about 0.18.

The stillness rests on five assumptions: no mutation, random mating, no gene flow, a huge population, and no selection. Real groups break them all the time. A small inbred group breaks the size and random mating assumptions at once, while migrants carry foreign alleles straight through the no gene flow wall.

**Example.** Use departures as your detector. If frequencies shift across generations, equilibrium is broken and something such as selection is pushing. Match each evolution force to the assumption it violates: mutation invents alleles, choosy mating reshuffles pairs, migration injects alleles, tiny groups drift by luck, and selection rewards winners. Spot the broken assumption and you have named the mechanism.

**Recap.** Still frequencies mean no evolution, and any lasting shift names a force that broke the assumptions.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Punnett Squares and Monohybrid Ratios](https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w)
- [Testing Genetic Ratios with Chi-squared](https://lightmysky.com/learn/science/testing-genetic-ratios-with-chi-squared-mt_YCO6jijrNQ)

## Opens up

- [Selection Coefficients and Predicting Allele Frequency Change](https://lightmysky.com/learn/science/selection-coefficients-and-predicting-allele-frequency-change-mt__JS2XJxOXV)
- [Recombinant DNA and Gene Transfer](https://lightmysky.com/learn/science/recombinant-dna-and-gene-transfer-mt_L9yncU_x2P)
- [Measuring Genetic Variation: Heterozygosity and Population Structure](https://lightmysky.com/learn/science/measuring-genetic-variation-heterozygosity-and-population-structure-mt_Prn7vgkyLl)