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Hardy-Weinberg Principle

Biology: Cambridge International A Level, topic 17.2.6

Using the Hardy-Weinberg Principle

The Hardy-Weinberg principle is used to predict allele frequencies in a population. The principle is a mathematical model.

Predictions

Predictions

  • The Hardy-Weinberg principle predicts that allele frequencies will not change from generation to generation.
  • The allele frequencies do not change because a population’s allele and genotype frequencies are inherently stable.
Assumptions

Assumptions

  • The Hardy-Weinberg principle only works if you make some assumptions about the population you are studying.
  • The assumptions are:
  • No mutations, migration or emigration.
  • No selective pressure for or against a specific genotype.
  • Population size is infinite.
  • In real populations, these assumptions are not normally met but making assumptions is still useful because it provides a model we can compare to real population changes.

The Hardy-Weinberg Equation

The Hardy-Weinberg equation is: p2 + 2pq + q2 = 1. The equation is used to predict the frequency of alleles, genotypes and phenotypes in a population.

p + q 1

p + q = 1

  • For every gene, there are two alleles present in an individual (except some genes on the X chromosome).
  • If you know the frequency of one of these alleles, the equation p + q = 1 can be used to calculate the frequency of the other allele.
  • p = frequency of one allele.
  • q = frequency of other allele.
p2 + 2pq + q2 1

p2 + 2pq + q2 = 1

  • The Hardy-Weinberg is equation: p2 + 2pq + q2 = 1
  • p2 = pp = homozygous dominant.
  • q2 = qq = homozygous recessive.
  • 2pq = heterozygous.
  • The components of the equation allows us to calculate the number of individuals in a population that have each genotype.
Predicting genotypes

Predicting genotypes

  • By observing the phenotypes in a population, you can identify how many individuals carry the homozygous recessive allele genotype (q2).
  • Only individuals who are homozygous recessive show the recessive phenotype.
  • Individuals with the dominant phenotype could be homozygous dominant or heterozygous.
  • This number can be used in the Hardy-Weinberg equation to allow the frequencies other two genotypes to be estimated.
Pea plants

Pea plants

  • Pea plants can have yellow peas (pp or pq) or green peas (qq).
  • If 0.25 of the population have green peas then q2 = 0.25.
  • If q2 = 0.25, then q = 0.5.
  • Using p + q = 1, p + 0.5 = 1, p = 0.5.
  • 2pq = 0.50.
  • p2 = 0.25.
  • p = 0.5.

Test yourself on Hardy-Weinberg Principle

  • Which of these is NOT an assumption made in the Hardy-Weinberg principle?

  • Which of the following statements about the Hardy-Weinberg equation are true?

  • Which genotype frequency can be easily identified by observing the phenotypes of the population?

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