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.