Allele Frequency Calculator
Estimate two allele frequencies and carrier prevalence from the frequency of a recessive phenotype.
On this page
CalculatorHow to useFormula & methodWorked exampleFrom phenotype to allele frequencyWhere the assumptions matterInterpreting a carrier estimateFrequently asked questionsRelated calculatorsSourcesHow to use this calculator
- Enter recessive phenotype prevalence as a percentage, not a decimal proportion.
- Confirm that a fully recessive two-allele model is suitable for the example.
- Read the inferred allele frequencies and expected carrier proportion.
Formula and method
For the assumptions behind these results, read the explanation below. You can also explore Hardy-Weinberg Equilibrium and Punnett Square.
A worked example
If 1% of a population shows a fully recessive phenotype, q² = 0.01. The inferred recessive allele frequency is 10%, the other allele frequency is 90%, and the expected heterozygote proportion is 18%.
From phenotype to allele frequency
For a fully recessive trait, two copies of the recessive allele are required to produce the modeled phenotype. Under Hardy–Weinberg assumptions, the fraction of individuals with that genotype is q². Taking its square root estimates the frequency q of the allele in the population’s gene pool.
Allele frequency is not the same as the fraction of individuals carrying an allele. Heterozygotes have one copy and recessive homozygotes have two. The frequency calculation counts copies across the population rather than counting only affected individuals.
Where the assumptions matter
The model concerns a population with two alleles at the locus. It assumes a genotype distribution consistent with random mating and no disruptive influence from processes such as selection, migration, or population subdivision. Real populations can depart from those conditions.
Using disease prevalence requires additional care: the disease must fit the inheritance model, and observed cases must represent the relevant population. Incomplete penetrance, age-dependent detection, multiple causes, and ascertainment can prevent prevalence from being a direct measure of q².
Interpreting a carrier estimate
The expected heterozygote proportion is 2pq. The factor of two represents the two parental arrangements that produce one copy of each allele. The remaining genotype proportions are p² and q², and all three sum to one.
These are population expectations rather than personal genetic test results. Knowing population prevalence alone does not establish an individual’s genotype, and it does not replace family information or an appropriate clinical assessment.
Frequently asked questions
Should I enter one percent as 1 or 0.01?
Enter 1. The form converts the percentage to the proportion 0.01 before taking the square root.
Can this be used for a dominant condition?
The square-root calculation here specifically uses a fully recessive phenotype. A different model is needed for dominant inheritance.
Why are carriers more common than affected individuals?
When q is small, 2pq can be much larger than q² because one recessive copy is far more frequent than two.
Sources and further reading
- OpenStax, Rice University — Biology 2e: Population Evolution ↗
Explains p + q = 1, p² + 2pq + q² = 1, and the assumptions needed for Hardy–Weinberg genotype expectations.
Our explanations are written for BiologyCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.