Allele Frequency
Frequencies of two alleles sum to one.
Formula
Variables
Example
Foundation of population genetics
How it works
In a simple gene with only two possible alleles, every copy of that gene in a population must be one allele or the other, so their frequencies must add up to the whole pool: 1 (or 100%). If one allele has frequency p, the only remaining fraction available for the other allele is q = 1 - p. This relationship is the basic bookkeeping rule behind many population genetics calculations.
When to use it
Use this when a trait is controlled by two alleles and you know the frequency of one but need the other. It is also useful as a quick check that your allele-frequency calculations are consistent and add to 1.
Worked example, step by step
In a population of beetles, the frequency of the dominant allele B is p = 0.72. Since there are only two alleles, p + q = 1. Substitute the known value: 0.72 + q = 1 Solve for q: q = 1 - 0.72 q = 0.28 So the recessive allele frequency is 0.28 (or 28%).
Common mistakes to avoid
- Adding allele frequencies and getting a total greater than 1
- Confusing allele frequency with genotype frequency
- Using this formula when more than two alleles are present
Did You Know?
Tracking allele frequencies over generations is how scientists actually watch evolution unfold in the wild.
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