Hardy-Weinberg’s principle states that the allele frequency of a population remains constant from generation to generation unless specific disturbing influences are introduced. This is called genotype equilibrium.

A single locus with two alleles: allele one is denoted as ‘A’ and allele two as ‘a’ and their frequencies are denoted by p and q;

$\begin{array}{l}\text{frequency }\left(\text{A}\right)=p\end{array}$
frequency $\begin{array}{l}\left(\text{a}\right)=q\end{array}$

And, $p+q=\text{ 1}$ .

If mating is random, then new individuals will have:

$$frequency $\left(\text{AA}\right)=p^2$ for the AA homozygotes in the population,

$$frequency $\left(\text{aa}\right)=q^2$  for the aa homozygotes, and

$$frequency $\left(\text{Aa}\right)=\text{ 2}pq$ for the Aa heterozygotes.