Class 12 Biology Evolution Hardy- Weinberg principle

Hardy- Weinberg principle

  • Hardy- Weinberg principle is also called genetic equilibrium.
  • Gene frequency remains constant from generation to generation and is stable, this is called genetic equilibrium.
  • Sum total of allelic frequencies is 1 and individual frequencies can be named as p,q etc. hence, p+q = 1, where P and q represent the frequency of allele A and allele a.
  • In diploids, the frequency of AA is p2, aa is q2 and of Aa is 2pq.

Hence, the formula is p2 + 2pq + q2.which is a binomial expansion of (p+q)2 which can be applied to any population to find out the gene frequency.

  • When frequency measured differs from expected value, the difference indicates the extent of evolutionary change.

Factors affecting Hardy- Weinberg principle are-

Gene flow –

  • The transfer of section of population to another place resulting in a change in gene frequencies in both old and new population is called gene flow.
  • New genes and alleles are added to new population which are genetically different but can interbreed.

Genetic drift-

  • The random change in gene frequency occurs by chance is called genetic drift.
  • Sometimes, the change in allelic frequency is so different in the new population,that they become a different species and the original drifted population becomes founders hence the effect is called founder effect.


  • The spontaneous change in the genetic makeup of an individual is called mutation.
  • Pre-existing advantageous mutations when selected will result in observation of new phenotypes and over few generations this would result in

Genetic recombination-

  • Exchange of genes between non sister chromatids of homologous chromosomes during gametogenesis is called genetic recombination.
  • Variation due to recombination during gametogenesis, or due to gene flow or genetic drift results in changed frequency of genes and alleles in future generation.

Natural selection-

  • The process by which better adapted individuals with useful variations are selected by nature and leave greater number of progenies is called natural selection.
  • Natural selection can lead to-
    • Stabilizing selection- here, more individuals acquire mean character value.
    • Directional change – here, more individuals acquire value other than the mean character value.
    • Disruptive selection- here, more individuals acquire peripheral character value at both ends of the distribution curve.

Problem based on Hardy- Weinberg principle

Question. The frequency of two alleles in a gene pool is 0.15 (A) and 0.75 (a). Assume that the population  is in Hardy-Weinberg equilibrium.

Calculate the percentage of heterozygous individuals in the population.

Solution- According to Hardy-Weinberg equilibrium equation, heterozygotes are represented by the 2pq term. Therefore, the number of heterozygous individuals (Aa) is equal to 2pq which equals 2x 0.15x 0.75 =0.225 = 22.5 %.

Calculate the percentage of homozygous recessives in the population.

Solution- The homozygous recessive individuals (aa) are represented by the q2 term which equals 0.75x 0.75=  0.562 = 56.25 %

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