<meta http-equiv="refresh" content="1; url=/nojavascript/"> Hardy-Weinberg Theorem | CK-12 Foundation
Skip Navigation
You are reading an older version of this FlexBook® textbook: CK-12 Biology Concepts Go to the latest version.

5.19: Hardy-Weinberg Theorem

Difficulty Level: At Grade Created by: CK-12
Practice Hardy-Weinberg Theorem
Practice Now

Why is balance important?

To these individuals, the importance of maintaining balance is obvious. If balance, or equilibrium, is maintained within a population's genes, can evolution occur? No. But maintaining this type of balance today is difficult.

The Hardy-Weinberg Theorem

Godfrey Hardy was an English mathematician. Wilhelm Weinberg was a German doctor. Each worked alone to come up with the founding principle of population genetics. Today, that principle is called the Hardy-Weinberg theorem . It shows that allele frequencies do not change in a population if certain conditions are met. Such a population is said to be in Hardy-Weinberg equilibrium . The conditions for equilibrium are:

  1. No new mutations are occurring. Therefore, no new alleles are being created.
  2. There is no migration . In other words, no one is moving into or out of the population.
  3. The population is very large.
  4. Mating is at random in the population. This means that individuals do not choose mates based on genotype.
  5. There is no natural selection. Thus, all members of the population have an equal chance of reproducing and passing their genes to the next generation.

When all these conditions are met, allele frequencies stay the same. Genotype frequencies also remain constant. In addition, genotype frequencies can be expressed in terms of allele frequencies, as the Table below shows. For a further explanation of this theorem, see Solving Hardy Weinberg Problems at http://www.youtube.com/watch?v=xPkOAnK20kw .

Hardy and Weinberg used mathematics to describe an equilibrium population ( p = frequency of A , q = frequency of a , so p + q = 1): p 2 + 2 pq + q 2 = 1. Using the genotype frequencies shown in Table below , if p = 0.4, what is the frequency of the AA genotype?

Genotype Genotype Frequency
AA p 2
Aa 2 pq
aa q 2

A video explanation of the Hardy-Weinberg model can be viewed at http://www.youtube.com/watch?v=4Kbruik_LOo (14:57).


  • The Hardy-Weinberg theorem states that, if a population meets certain conditions, it will be in equilibrium.
  • In an equilibrium population, allele and genotype frequencies do not change over time.
  • The conditions that must be met are no mutation, no migration, very large population size, random mating, and no natural selection.

Explore More

Use this resource to answer the questions that follow.

  1. This resource states that evolution will not occur in a population if seven conditions are met. What are these seven conditions?
  2. If there is no evolution, what happened to gene frequencies?
  3. What are p and q?
  4. How is p determined?
  5. p + q = _____
  6. p 2 + 2pq + q 2 = _____


  1. Describe a Hardy-Weinberg equilibrium population. What conditions must it meet to remain in equilibrium?
  2. Assume that a population is in Hardy-Weinberg equilibrium for a particular gene with two alleles, A and a . The frequency of A is p , and the frequency of a is q . Because these are the only two alleles for this gene, p + q = 1.0. If the frequency of homozygous recessive individuals ( aa ) is 0.04, what is the value of q ? What is the value of p ?
  3. Use the values of p and q from question 2 to calculate the frequency of the heterozygote genotype ( Aa ).


Hardy-Weinberg equilibrium

Hardy-Weinberg equilibrium

State where allele frequencies do not change in a population.
Hardy-Weinberg theorem

Hardy-Weinberg theorem

Founding principle of population genetics; proves allele and genotype frequencies do not change in a population that meets the conditions of no mutation, no migration, large population size, random mating, and no natural selection.


Regular movement of individuals or populations each year during certain seasons, usually to find food, mates, or other resources.

Image Attributions


Difficulty Level:

At Grade


Date Created:

Feb 24, 2012

Last Modified:

Jan 25, 2015
Files can only be attached to the latest version of Modality


100 % of people thought this content was helpful.
( 0 )
Most Helpful Reviews:
Loading reviews...
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original

Original text