Hardy-Weinberg principle

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In genetics, the Hardy–Weinberg principle states that gene frequency remains constant across generations (no genetic drift) unless a force causes phenomena such as gene flow.

In an example of two alleles, 'A' and 'a', random frequency is based on the binomial expansion

Thus the following distributions are examples of equilibrium:

  • AA is 100%; Aa and aa are both 0%
  • aa is 100%; Aa and AA are both 0%
  • Aa is 50%; AA and aa are both 25%
  • Aa is 4/9; AA is 4/9 and aa is 1/9
  • Aa is 4/9; AA is 1/9 and aa is 4/9

Testing for Hardy–Weinberg equilibrium is studies of associations between genetic polymorphisms and diseases is controversial.[1][2] Studies having violations of equilibrium may have less stable results and may tend to have more positive results.[2]


  1. Zou GY, Donner A (2006). "The merits of testing Hardy-Weinberg equilibrium in the analysis of unmatched case-control data: a cautionary note.". Ann Hum Genet 70 (Pt 6): 923-33. DOI:10.1111/j.1469-1809.2006.00267.x. PMID 17044866. Research Blogging.
  2. 2.0 2.1 Trikalinos TA, Salanti G, Khoury MJ, Ioannidis JP (2006). "Impact of violations and deviations in Hardy-Weinberg equilibrium on postulated gene-disease associations.". Am J Epidemiol 163 (4): 300-9. DOI:10.1093/aje/kwj046. PMID 16410351. Research Blogging.