Hardy-weinberg Equilibrium

Unit: Natural selection

Chapter: Hardy Weinberg equilibrium

Reference: Hardy-Weinberg principle, Importance

Learning objectives

  • To describe the conditions under which allele and genotype frequencies will change in populations.
  • To explain the impacts on the population if any of the conditions of Hardy-Weinberg are not met.

Hardy-Weinberg principle

  • Coined by G. H. Hardy and W. Weinberg, which describes the genetic structure of evolving population.
  • Mutations introduce new genes into species resulting a change in gene frequencies.
  • Gene frequency is defined as the frequency with which a particular allele occurs in the population.

According to Hardy-Weinberg principle allele frequencies in a population are stable and is constant from generation to generation i.e., the gene pool remains constant, called genetic equilibrium.

According to this principle a population is only at equilibrium when there is-

  • No mutation
  • No natural or artificial selection
  • No gene flow (no gene migration)
  • No genetic drift
  • No genetic recombination

Individual frequency for may be named as P, q. etc. in a diploid organism p and q represent the frequency of allele A and a. the frequency of AA individuals in a population is p2. This can be stated in another way, i.e., the probability that an allele A with a frequency of p appears on both the chromosome of a diploid individual is the product of the probabilities i.e., p2. Similarly, of aa is q2, of Aa is 2 pq. Thus, is a binomial expansion of (p+q)2. It is possible to calculate all allele and genotype frequencies using the expressions allele frequency p+q = 1 and genotype frequency p2+2pq+q2=1

Importance

For populations in the Hardy-Weinberg equilibrium, the equation connects allele frequencies to genotype frequencies. This equation allows us to compare a population's genetic structure over time to the expected genetic structure if the population were in Hardy-Weinberg equilibrium.

Solved examples

Example 1. If the frequency of non-tasters of PTC – Paper in a population is 16%, find out the frequency of dominant allele and the percentage of homozygous dominant individuals in the population.

 Solution 1– The frequency of non-tasters (i.e., homozygous recessive genotype) is 16%,

this means that q2 = 16% = 16/100 = 0.16

Value of q = 0.16, = 0.4.

As p + q = 1, p = (1 – 0.4) = 0.6

Frequency of dominant allele (p) will, therefore, be 0.6 or 60%.

Now, the frequency of homozygous dominant genotype (p2) = 0.6 × 0.6 = 0.36, or 36%

Example 2. Which Hardy-Weinberg factor represents the frequency of heterozygous individuals in a population?

a) p2   b)2pq   c) q2   d) p2 + 2pq

Solution 2. b. The heterozygous individuals in a population are 2pq

Summary

  • Hardy-Weinberg principle is coined by G. H. Hardy and W. Weinberg, which describes the genetic structure of evolving population.
  • According to Hardy-Weinberg principle allele frequencies in a population are stable and is constant from generation to generation i.e., the gene pool remains constant, called genetic equilibrium.
  • The factors that affect the Hardy Weinberg principle includes gene flow, genetic drift, mutation, genetic recombination, and natural selection.

 

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