Allele Frequency Calculator: Formula and Examples

This calculator works out the frequency of one allele in a population from the number of copies counted. Enter the allele count and the total number of allele copies; the result is a proportion from 0 to 1, with its percentage equivalent.

Formula

Allele frequency is the number of copies of the allele being studied divided by the total number of allele copies at that genetic location:

f(A) = count of A copies / total allele copies

In the calculator, a is the number of copies of the target allele, measured in copies; n is the total number of copies of all alleles at the same locus in the sampled population, also measured in copies; f is the allele frequency, a unitless proportion; and p is that frequency expressed as a percentage. The denominator must be at least as large as the target-allele count. A frequency of 0.4 is equivalent to 40%.

For a diploid organism, each individual usually contributes two allele copies at an autosomal locus. If all individuals are successfully genotyped, the total is twice the number of individuals. For a haploid organism, each contributes one copy. Sex-linked loci, organisms with different ploidy, and missing genotype data require counting the actual copies represented rather than automatically multiplying by two.

Worked Example

Using the calculator’s default values, the target allele has 40 copies and the sample has 100 allele copies in total.

1. Divide the target count by the total: 40 / 100 = 0.4.
2. Convert the proportion to a percentage: 0.4 × 100 = 40%.
3. Report the allele frequency as 0.400, or equivalently 40.0%.

The proportion is shown to three decimal places and the percentage to one, as set in the calculator. Keep the unrounded counts for calculations and round only the displayed result.

What You’ll Need

• The number of copies of the allele whose frequency you want to calculate.
• The total number of allele copies counted at that same locus.
• A clear definition of your sample, locus, allele, and any exclusions for missing or uncertain genotypes.
• For genotype-count data, the ploidy and genotype counts needed to convert individuals into allele copies.

Step-by-Step

1. Define the population and locus. Decide which individuals or samples are included and identify the specific gene location and allele. Allele frequency refers to a particular allele at a particular locus in a defined population or sample; it is not the frequency of a trait in general.
2. Choose the target allele. For a gene with alleles A and a, for example, decide whether you are calculating f(A) or f(a). In a two-allele system, the frequencies should add to 1 when both alleles are counted in the same complete sample.
3. Count the target allele copies. If you have genotype counts for a diploid autosomal locus, each AA individual contributes two A copies, each Aa individual contributes one A copy, and each aa individual contributes none. Therefore, count A copies as 2 × the number of AA individuals plus the number of Aa individuals.
4. Count all observed copies at the locus. With complete diploid genotypes, multiply the number of individuals by two. If some calls are missing, exclude those individuals from both the target count and total copy count. For haploid or other ploidy systems, use the number of copies each included sample actually carries.
5. Check the counts. Confirm that the target count is zero or greater and does not exceed the total. Make sure both counts refer to the same locus, population, and set of successfully observed samples. Do not put genotype counts in the calculator where allele-copy counts are requested.
6. Enter the values. Put the target allele-copy count in “Copies of target allele” and the total allele-copy count in “Total allele copies.” The calculator requires a positive total so the division is defined.
7. Read and report both forms. The first result is the frequency as a proportion; the second is the percentage. Include the allele name and sample context in your notes, such as “A frequency = 0.40 (40%) among the 50 successfully genotyped individuals.”
8. For multiple alleles, repeat the calculation. Calculate each allele’s copy count divided by the same total. In a complete sample, the resulting frequencies should sum to 1, subject to rounding.

Common Mistakes

• Using individuals as the denominator. For diploid autosomal data, the denominator is normally allele copies, not people. Fifty fully genotyped individuals contribute 100 copies.
• Counting heterozygotes incorrectly. An Aa individual contributes one A and one a, not two copies of either allele.
• Mixing samples or loci. The target count and total must come from the same population, locus, and set of observations.
• Handling missing genotypes inconsistently. Do not leave missing samples in the denominator if their alleles were not observed. Exclude their copies from both numerator and denominator.
• Confusing allele frequency with genotype frequency. Genotype frequency is the proportion of individuals with a genotype; allele frequency is the proportion of allele copies that are a given allele.
• Rounding too early or using percent in the wrong field. Enter counts, not a frequency or percentage. Calculate with the original counts and round the final result.

Tips

• Write down the genotype-to-copy rule before counting, especially when there are more than two alleles or organisms differ in ploidy.
• Keep a table with one column per allele. For diploid genotypes, each individual should contribute two allele copies in total if the genotype is complete.
• Use the proportion for equations that expect values from 0 to 1, and use the percentage for a reader-friendly summary. Label which form you report.
• Check that frequencies across all alleles sum approximately to 1. A small difference can result from rounding; a substantial difference suggests missed copies, duplicate counting, or inconsistent denominators.
• When comparing populations, report sample sizes and how missing or uncertain calls were treated. Sampling estimates can differ even when the underlying populations are similar.

Troubleshooting

The calculator rejects the values: Check that both entries are counts, the total is positive, and the target count is no greater than the total. Enter 40 rather than 0.4 if you observed 40 copies out of 100.

The result is above 1 or above 100%: The target count is likely larger than the denominator, or the two counts use different units or samples. Recount allele copies and verify the total.

The frequencies do not sum to 1: Check that every allele at the locus has been included, every observed copy is counted once, and missing data are removed consistently. If results are rounded, compare unrounded frequencies before treating a small discrepancy as an error.

You have genotype counts but not allele counts: Convert each genotype into copies of each allele first. For example, in a diploid two-allele sample, AA contributes two A copies, Aa contributes one A and one a, and aa contributes two a copies.

The locus is sex-linked or has unusual ploidy: Do not assume two copies per individual. Establish how many copies each included sample can carry at that locus, then sum the observed copies for the denominator.

Frequently Asked Questions

What is allele frequency?
It is the proportion of all copies of a particular allele at a locus that are that allele, in a defined population or sample.

Can allele frequency be greater than 1?
No. As a proportion it ranges from 0 to 1, or from 0% to 100%. A value outside that range indicates a counting or denominator error.

How do I calculate allele frequency from genotype frequencies?
For diploid A/a data, f(A) = frequency(AA) + 0.5 × frequency(Aa). Equivalently, count A copies and divide by all copies. The same method works for the other allele.

Do allele frequencies have units?
No. They are proportions, so they are unitless. They can also be reported as percentages.

Should missing genotypes be included?
Not in the allele-copy denominator if their alleles were not observed. Use the successfully observed copies for both numerator and denominator, and state how many samples were excluded when relevant.

Does allele frequency tell me how common a trait is?
Not by itself. A trait may depend on genotype, dominance, environment, or multiple genes. Allele frequency describes copies of an allele, not directly the proportion of people showing a trait.

Related

• Generation Time Formula & How to Calculate It
• Allele Frequency G5: Step-by-Step Calculation
• Confidence Interval Formula: How to Calculate

in a | Copies of target allele | copies | 40 | 0
in n | Total allele copies | copies | 100 | 1
out f | Allele frequency | proportion | a / n | 3
out p | Allele frequency | percent | f * 100 | 1