Calculating allele frequencies

Generally when working with the H-W equation, you are given the information on your population and then must calculate the required values. The first set of values you are likely to need are the allele frequencies (p for the dominant, q for the recessive). To determine these values you will be given the number of individuals in a population for each genotype. From this you can calculate the values directly. For example, if you have a population with 252 AA, 640 Aa, and 382 aa. What are the values of p and q for this population? Because p and q represent the allele frequencies in the gene pool, you need to determine how many alleles are in the entire gene pool. For this you take the total number of individuals and multiply by 2 (because the individuals are diploid). Thus:

252 + 640 + 382 = 1274 individuals.

1274 x 2 = 2548 alleles

To calculate an allele's frequency you need to take into account how many total copies are present for that allele in the entire gene pool. This means you need to take into account two different types of individuals: heterozygotes and one type of homozygous individual. If you are calculating p, then you would count all the homozygous dominant individuals and the heterozygotes. However, you have to take into account the fact that a homozygous dominant individual has two dominant alleles while heterozygotes have only one. So in order to calculate p, you need to take

 (equation 1)

Replacing the generic equation listed above with the actual values we get 

 (equation 2)

This means that p = 0.449 (your number may vary slightly due to different number of decimal places). This means you know that q must be 0.551, because p + q = 1. You can also calculate q directly as a way to check your math (always a good idea!). You would use equation 2, but replace 252 with 382.

This is an important skill to cultivate - pretty much every other calculation you need for the Hardy-Weinberg equation requires that you know what the allele frequencies are!