Genotype Frequency Calculator
Enter observed counts of each genotype (AA, Aa, aa) in a population to compute their genotype frequencies and the derived allele frequencies (p and q). Two 3D diagrams compare a population dominated by one genotype to a more evenly mixed one, and charts show how genotype counts translate into frequencies and allele proportions.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter observed counts for each genotype
Skewed vs. mixed population
1. Dominant-heavy population
Most individuals are homozygous dominant — a high AA count relative to the others.
2. Evenly mixed population
All three genotypes present in more comparable numbers.
Genotype and allele frequency charts
How it works
The core idea in one line: counting how many individuals have each genotype and dividing by the population size gives genotype frequency directly — no assumptions required — and carefully counting each individual allele copy from those same genotypes gives allele frequency.
freq(genotype) = count / N
N = total population size
p = freq(AA) + ½freq(Aa)
dominant allele frequency from genotype counts
q = freq(aa) + ½freq(Aa)
recessive allele frequency from genotype counts
Genotype frequency is simply the fraction of a population carrying a specific genotype, calculated by dividing that genotype's count by the total population size — a purely observational measurement requiring no theoretical assumptions. Allele frequency takes this one step further: since homozygotes (AA or aa) contribute two identical allele copies each, and heterozygotes (Aa) contribute one copy of each allele, tallying every allele copy across the whole population and dividing by the total number of alleles gives p and q. These observed frequencies become genuinely powerful once compared against Hardy-Weinberg's theoretical predictions — any mismatch is direct evidence that some evolutionary force is acting on the population.
Worked example 1 — a sampled population of 100
Given: A field survey counts 60 AA, 30 Aa, and 10 aa individuals (N = 100).
Notice p+q=0.75+0.25=1.0 exactly, as required — this is a useful built-in check that the counts were entered and computed correctly.
Worked example 2 — a smaller sample of 50
Given: A smaller sample counts 32 AA, 15 Aa, and 3 aa individuals (N = 50).
A smaller sample size gives a slightly different result even from a similar underlying population — larger samples generally give more statistically reliable frequency estimates.
From genotype counts to allele frequency, step by step
Each row uses the population from worked example 1 (N=100).
| Quantity | Formula | Value |
|---|---|---|
| freq(AA) | 60/100 | 0.60 |
| freq(Aa) ★ | 30/100 | 0.30 |
| freq(aa) | 10/100 | 0.10 |
| p (dominant allele) | 0.60 + ½(0.30) | 0.75 |
| q (recessive allele) | 0.10 + ½(0.30) | 0.25 |
★ Reference row. Heterozygotes contribute to both allele frequencies equally, which is exactly why the ½freq(Aa) term appears in both the p and q formulas.
Where genotype frequency actually matters
🔬 Population genetics field surveys
Ecologists and geneticists sampling wild populations (plants, insects, fish) count genotypes directly from field or lab data — this is the very first step before any deeper population genetics analysis can happen.
🧬 Genetic disease screening programs
Public health genetics programs track genotype frequencies for known disease alleles across populations, using exactly this calculation to estimate carrier rates and plan screening resources.
🐄 Livestock and crop genetic monitoring
Breeders and agricultural geneticists track genotype frequencies across generations to monitor whether a desirable or undesirable allele is becoming more or less common in a managed population.
🦋 Studying evolution in real time
Comparing genotype frequencies of the same population measured in different years is one of the most direct ways biologists document evolution actually happening, such as shifts in coloration alleles in response to environmental change.
Common misconceptions
"Genotype frequency and allele frequency are the same number."
They measure different things — genotype frequency is the proportion of individuals with a specific genotype (like Aa), while allele frequency is the proportion of a specific allele among all alleles in the population's gene pool. They're related but numerically different.
"You need to assume Hardy-Weinberg equilibrium to calculate genotype frequency."
Genotype frequency is simply an observed proportion from real counts — no equilibrium assumption is needed at all. Hardy-Weinberg equilibrium only becomes relevant when you want to predict or compare against a theoretical expectation.
"A rare genotype in a sample means it's rare in the whole population."
A small sample can easily over- or under-represent a genotype's true population frequency by chance — larger, more representative samples are needed before drawing confident conclusions about the actual population.
"Allele frequency can be read directly from genotype counts without any calculation."
Allele frequency must be calculated by counting actual allele copies — homozygotes contribute two copies of the same allele and heterozygotes contribute one of each, which is why the ½freq(Aa) term is essential and can't be skipped.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Biology 2e — Chapter 19, "Population and Community Ecology" and Chapter 20, "Evolution" (free, peer-reviewed). openstax.org
- • Hartl & Clark, Principles of Population Genetics — Allele and genotype frequency chapter.
- • Campbell & Reece, Biology — Population genetics and Hardy-Weinberg chapter.
freq = count/N; p = freq(AA)+½freq(Aa); q = freq(aa)+½freq(Aa). Results are rounded for display.
How to use this calculator
Enter genotype counts
Type the number of AA, Aa, and aa individuals observed.
Read genotype frequencies
Each genotype's proportion of the total population displays instantly.
Read allele frequencies
p and q are derived automatically — they should always sum to 1.
Related tools
Frequently asked questions
What is genotype frequency?
Genotype frequency is the proportion of a population with a specific genotype — for example, if 60 out of 100 individuals are AA, the genotype frequency of AA is 0.60 (60%).
How is this different from the Hardy-Weinberg Allele Frequency Calculator?
The Hardy-Weinberg Allele Frequency Calculator starts from a known or assumed allele frequency and predicts what genotype frequencies should look like at equilibrium. This calculator instead starts from real, observed genotype counts and computes the actual frequencies directly — no equilibrium assumption required.
How do I get allele frequency from genotype frequency?
Each homozygote contributes two copies of its allele, and each heterozygote contributes one copy of each — so allele frequency p = freq(AA) + ½freq(Aa), and q = freq(aa) + ½freq(Aa).
Do genotype frequencies always add up to 1?
Yes — every individual in the population must have exactly one of the possible genotypes, so freq(AA) + freq(Aa) + freq(aa) always equals 1 (100%) by definition.
Can I use this to check if a population is in Hardy-Weinberg equilibrium?
This calculator gives you the observed genotype and allele frequencies — comparing those observed genotype frequencies to the Hardy-Weinberg-predicted p², 2pq, q² (using the same p and q) is exactly how equilibrium is tested, typically with a chi-square goodness-of-fit test.