Hardy-weinberg Equilibrium

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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.