Hardy-Weinberg principle

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

$100\%=A^{2}+2\times A\times a+a^{2}$

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.