Allele Frequency Calculator

Calculate carrier rates using Hardy-Weinberg equilibrium

Enter disease prevalence to find allele frequencies, or input any value to reverse-calculate.

Last updated: December 14, 2025

CreatorFrank Zhao

Frequency of the disease in the population

Occurrence of the disease:

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1 in

Allele frequency

Dominant allele frequency (p)

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Recessive allele frequency (q)

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What is allele frequency?

Think of allele frequency as a snapshot of how common different versions of a gene are in a population. If you have a town of 10,000 people and 4,000 of them carry a recessive allele for a particular gene, that allele has a frequency of 40% in your town.

💡 The key insight: Everyone carries two copies of each gene — one from mom, one from dad. So a population's genetic mix tells us a lot about inherited disease risk.

This is especially important for recessive diseases like cystic fibrosis or sickle cell anemia. You might be a "carrier" without ever getting sick, but if your partner is also a carrier, your children are at risk.

💭 What's an allele?

A variant of a gene. You inherit one version from each parent.

👥 What's a population?

Any group sharing similar ancestry—a country, ethnic group, or even a town.

🧫 Why it matters?

Disease prevalence varies wildly between populations. Cystic fibrosis is far more common in Caucasians than East Asians.

👨‍👩‍👧‍👦 Carrier risk?

Two carrier parents have a 25% chance of having an affected child each pregnancy.

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How to use the allele frequency calculator

The calculator works in both directions. You can start with disease frequency and calculate allele frequencies, or vice versa. Here's what you need to know:

Choose your input format

Is disease prevalence given as a percentage (0.05%) or proportion (1 in 2,000)? Pick the matching option.

Enter the disease frequency

If you know how common the disease is in the population, enter that number. The calculator derives everything else.

Or enter the allele frequencies directly

If you already know p (healthy allele) or q (disease allele), enter those instead. The calculator reverses the calculation.

Read your results

The calculator shows the genotype frequencies (how common each genetic combination is) and carrier frequency (1 in X people carries the mutation).

💡 Pro tip: The calculator uses the Hardy-Weinberg equation, which assumes no mutation, migration, or selection pressure. For most populations in equilibrium, this works great. See the limitations section for when to be cautious.

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Quick start guide: A real example

Let's say you read that cystic fibrosis affects 1 in 2,500 Caucasians. You want to know: what's the chance you're a carrier?

Step 1: Set up

Disease frequency = 1 in 2,500 (or 0.04%)

Step 2: Calculate

q² = 1/2500 = 0.0004

q = √0.0004 = 0.02

p = 1 - 0.02 = 0.98

Step 3: Results

Carriers (2pq) = 2 × 0.98 × 0.02 = 0.0392 = 3.92%

About 1 in 25 Caucasians carry a CF mutation.

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What this means:

If you're Caucasian and both you and your partner are carriers (unlikely but possible), each child has a 25% chance of having cystic fibrosis. If only one of you is a carrier, your kids won't have the disease but might be carriers themselves.

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Real-world examples & use cases

🫁 Cystic Fibrosis

In Caucasians:

Affects ~1 in 2,500. Carrier rate: ~1 in 25.

In East Asians:

Much rarer (~1 in 30,000). Carrier rate: ~1 in 174.

This is why genetic counseling should consider ethnicity.

🩸 Sickle Cell Anemia

In African-Americans:

Affects ~1 in 500. Carrier rate: ~1 in 10.

In West Africans:

Even higher—about 1 in 3 carries the trait (protective against malaria).

The mutation is actually beneficial in malaria-endemic areas.

👨‍👩‍👧 Tay-Sachs

In Ashkenazi Jews:

Affects ~1 in 3,600. Carrier rate: ~1 in 30.

In general population:

About 100 times rarer.

Carrier screening is standard in high-risk communities.

🔄 Heterozygote Advantage

The twist:

Sickle cell carriers (Aa) have malaria resistance but don't have sickle cell disease. This is why the allele persists.

Evolution in action:

Allele frequencies can tell us when natural selection is at work.

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Understanding the Hardy-Weinberg equation

The Hardy-Weinberg equation is the foundation of population genetics. It tells us that in a population with no forces acting on it (no new mutations, no migration, no selection), allele frequencies stay constant from generation to generation.

Core equation

p² + 2pq + q² = 1

Where:

• p = frequency of dominant (healthy) allele
• q = frequency of recessive (disease) allele
• p + q = 1 (they must add to 100%)
• p² = homozygous dominant (AA) — healthy, two healthy alleles
• 2pq = heterozygous (Aa) — carriers, one healthy + one disease allele
• q² = homozygous recessive (aa) — affected, two disease alleles

Why does this matter?

Predictability: If you know one frequency, you can calculate all the others. That's the whole basis of this calculator.
Equilibrium check: If a real population doesn't match Hardy-Weinberg predictions, something interesting is happening—selection pressure, migration, inbreeding, etc.

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Historical note:

Developed independently by G.H. Hardy (mathematician) and Wilhelm Weinberg (physician) in 1908, this equation has been fundamental to genetics ever since. It's one of the most elegant ideas in biology.

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Tips & best practices

1. Know your population

Disease prevalence varies hugely by ancestry. Cystic fibrosis in Caucasians looks very different from Asians. Always specify the population when discussing carrier risk.

2. Remember carrier risk doesn't mean disease risk

A carrier has one healthy + one disease allele. They're usually fine (some exceptions like sickle cell trait). Risk comes when both parents are carriers.

3. Use the calculator bidirectionally

Start with disease frequency OR allele frequencies—whichever you know. The calculator solves for everything else. This flexibility makes it powerful.

4. Watch for rounding

With very rare diseases, small rounding errors can matter. The calculator handles precision well, but if you're doing manual calculations, be careful with significant figures.

5. Consult a genetic counselor for real decisions

This calculator is educational and informative. If you're considering genetic testing or family planning decisions, talk to a healthcare professional who understands your specific situation.

6. Check your assumptions

Hardy-Weinberg assumes the population is large, mating is random, and there's no mutation or migration. If these aren't true (like in small isolated communities), the calculator is less accurate.

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Common recessive diseases at a glance

Disease	High-risk group	Prevalence	Carrier risk
Cystic Fibrosis	Caucasians (Northern Europe)	1 in 2,500	~1 in 25
Sickle Cell Anemia	African-Americans	1 in 500	~1 in 10
Tay-Sachs	Ashkenazi Jews	1 in 3,600	~1 in 30
Phenylketonuria (PKU)	Caucasians	1 in 15,000	~1 in 60
Albinism	General population	1 in 10,000	~1 in 100

Note: Frequencies vary by geographic region and ethnic background. These are approximate values for the populations listed. Always consult with a genetic counselor for personalized risk assessment.

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Frequently asked questions

▶Can I be a carrier without knowing it?

Absolutely. Carriers typically have no symptoms because one healthy allele is usually enough. The only way to know for sure is genetic testing. If there's a family history of a genetic disease, carriers are more likely.

▶If one parent is a carrier and the other isn't, can kids get the disease?

Not from two carrier parents, no. If one parent is a carrier (Aa) and the other is homozygous dominant (AA), all kids are either AA or Aa—they won't have the disease, but they might be carriers.

▶Does Hardy-Weinberg always hold true?

In theory, yes—in populations that are large, randomly mating, with no mutation/migration/selection. In reality, small populations, inbreeding, geographic isolation, or disease-causing alleles under selection can violate it. It's a useful approximation, not a law of nature.

▶How do I know which disease frequency to use for my population?

Check published epidemiological data for your ancestry. Different ethnic groups have different frequencies. Your doctor or genetic counselor can help you find accurate numbers.

▶What's the difference between p and q?

In this calculator, p is the healthy (dominant) allele frequency and q is the disease (recessive) allele frequency. They must add to 1 (100%) because those are the only two variants.

▶Why is the carrier frequency (2pq) so much higher than disease frequency (q²)?

Because carriers have one of each allele (2pq), while affected individuals have two disease alleles (q²). When an allele is rare, heterozygotes vastly outnumber homozygotes. This is why screening carriers is so important—they're far more common than affected individuals.

▶Can the same disease have different alleles?

Yes! Cystic fibrosis has hundreds of known mutations. This calculator treats the disease as a single recessive trait, but in reality, different mutations can cause the same disease with varying severity.

▶Is this calculator suitable for medical decisions?

It's excellent for understanding genetics and education. But for actual medical decisions—family planning, genetic testing, risk assessment—you should talk to a genetic counselor or doctor who can factor in your complete personal and family history.

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Limitations & disclaimers

🏥 Medical Disclaimer

This calculator is for educational purposes only and should not replace professional medical advice. Genetic risk assessment, carrier screening decisions, and prenatal diagnosis require consultation with a qualified genetic counselor or healthcare provider who understands your specific situation, family history, and medical context.

What this calculator assumes:

• Large population (to minimize random genetic drift)
• Random mating (no inbreeding)
• No new mutations
• No migration
• No natural selection (all genotypes survive equally)
• Autosomal recessive inheritance (not X-linked or dominant)
• Only two alleles per gene (not accounting for rare variants)

When to be cautious:

• Small populations or isolated communities (may not follow Hardy-Weinberg)
• Diseases with many different mutations (calculator treats as single variant)
• X-linked or dominant diseases (different inheritance patterns)
• Complex genetic conditions influenced by multiple genes
• Incomplete penetrance or variable expressivity

💡 Best practice: Use this calculator to understand genetics concepts and get approximate risks. Always verify disease prevalence data from reputable sources (CDC, WHO, peer-reviewed studies) and discuss results with a healthcare professional.