---
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. A departure from that baseline is evidence that one or more evolutionar"
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: 2026-11-12
---

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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. A departure from that baseline is evidence that one or more evolutionary forces may be acting.

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.
- Lists the conditions the model assumes and names a real population that breaks one of them.
- Explains why a change in allele frequencies across generations is evidence of evolution and may point to selection or another force.

## Lesson: When gene pools stand still

*(drawing: The two Hardy-Weinberg equations describe allele and genotype frequencies.)*

**Example.** In equilibrium, p and q stay the same from one generation to the next. This baseline model expects no mutation, random mating, no gene flow, a very large population, and no selection.

The model assumes no mutation, random mating, no gene flow, a very large population, and no selection. A small inbred group breaks the size and random-mating conditions. The Afrikaner population of South Africa is a real case: it grew from a small group of founders, so it broke the very-large-population condition, and alleles carried by a few founders became unusually common.

To find an allele frequency from genotype counts, count copies. AA gives two A copies, Aa gives one, and aa gives none. Divide A copies by all copies at the gene.

**Example.** For 30 AA, 40 Aa, and 30 aa, there are 60 plus 40, or 100 A copies. There are 200 total copies, so p is 100 divided by 200, or 0.5.

If the condition is fully recessive and the population follows the model, affected people are aa. Four affected people in 100 gives q² = 0.04, q = 0.2, p = 0.8, and carrier frequency 2pq = 0.32.

**Example.** A lasting frequency change means the population has evolved at that gene. The cause might be selection, drift, mutation, migration, or nonrandom mating. More evidence is needed before naming one force.

**Recap.** Hardy-Weinberg gives a still baseline. Count allele copies, use q squared carefully, and treat a frequency change as evidence of evolution rather than proof of one cause.

*Original lesson, assessment, and flashcard text authored for this topic in the Open Source Taxonomy project.*

## Practice

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

## Needs first

- [Testing Genetic Ratios with Chi-squared](https://lightmysky.com/learn/science/testing-genetic-ratios-with-chi-squared-mt_YCO6jijrNQ)

## Optional connections

- [Punnett Squares and Monohybrid Ratios](https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w)

## Opens up

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