5.19: HardyWeinberg Theorem
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 HardyWeinberg 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 HardyWeinberg 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 HardyWeinberg equilibrium. The conditions for equilibrium are:
 No new mutations are occurring. Therefore, no new alleles are being created.
 There is no migration. In other words, no one is moving into or out of the population.
 The population is very large.
 Mating is at random in the population. This means that individuals do not choose mates based on genotype.
 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.
Genotype  Genotype Frequency 

AA  p^{2} 
Aa  2pq 
aa  q^{2} 
Hardy and Weinberg used mathematics to describe an equilibrium population (p = frequency of A, q = frequency of a): p^{2} + 2pq + q^{2} = 1. In Table above, if p = 0.4, what is the frequency of the AA genotype?
A video explanation of the HardyWeinberg model can be viewed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/14/4Kbruik_LOo (14:57).
Summary
 The HardyWeinberg 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.
Practice
Use this resource to answer the questions that follow.

http://www.hippocampus.org/Biology
→ Biology for AP*→ Search: Population Genetics
 Describe the HardyWeinberg equation. What is the equation?
 What are the requirements for a population to be at equilibrium?
 What do p^{2}, q^{2} and 2pq refer to?
Review
1. Describe a HardyWeinberg equilibrium population. What conditions must it meet to remain in equilibrium?
2. Assume that a population is in HardyWeinberg 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? Based on the value of q, find p. Then use the values of p and q to calculate the frequency of the heterozygote genotype (Aa).
HardyWeinberg equilibrium
State where allele frequencies do not change in a population.HardyWeinberg 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.migration
Regular movement of individuals or populations each year during certain seasons, usually to find food, mates, or other resources.Image Attributions
 State the HardyWeinberg theorem.