Allele Frequency Sum
The two allele frequencies must total 1.
Formula
Variables
Example
p=0.7 ⇒ q=0.3
How it works
In a simple two-allele model, a gene can exist in only two forms at a particular locus, so the fraction of the population carrying one allele plus the fraction carrying the other must make up the whole population. That is why the frequencies add to 1: if one allele becomes more common, the other must become less common by the same amount. This relationship is a basic piece of population genetics and is often used as part of Hardy-Weinberg calculations.
When to use it
Use this when you know one allele frequency and need the other in a two-allele system, such as studying dominant and recessive alleles in a population. It is also useful as a quick check that allele-frequency data are valid and sum to 100%.
Worked example, step by step
Suppose the frequency of the dominant allele is p = 0.42. Use the formula p + q = 1. Substitute the known value: 0.42 + q = 1. Solve for q: q = 1 - 0.42. q = 0.58. So the recessive allele frequency is 0.58.
Common mistakes to avoid
- Adding genotype frequencies (like p^2, 2pq, q^2) instead of allele frequencies
- Forgetting that p and q must sum to 1, not 100 unless using percentages
- Using this formula for more than two alleles without adjusting the model
Did You Know?
Because allele frequencies must add to 1, knowing one frequency instantly gives you the other.
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