To estimate the frequency of alleles in a population, we can use the Hardy-Weinberg equation. According to this equation:

*p* = the frequency of the dominant allele (represented here by *A*)

*q* = the frequency of the recessive allele (represented here by *a*)

For a population in genetic equilibrium:

*p* + *q* = 1.0 (The sum of the frequencies of both alleles is 100%.)

(*p* + *q*)^{2} = 1

so

*p*^{2} + 2*pq* + *q*^{2} = 1

The three terms of this binomial expansion indicate the frequencies of the three genotypes:

*p*^{2} = frequency of *AA* (homozygous dominant)

2*pq* = frequency of *Aa* (heterozygous)

*q*^{2} = frequency of *aa* (homozygous recessive)

This page contains all the information you need to calculate allelic frequencies when there are two different alleles. You may want to print the page so you can refer back to it as you do the rest of this laboratory.

We start with some sample problems that will give you practice in using the Hardy-Weinberg equation.