punnett calculator

What Is a Punnett Calculator?

A Punnett calculator is a fast way to predict the probability of offspring genotypes and phenotypes from two parents. It is based on the classic Punnett square, a grid developed to model how alleles are inherited under Mendelian genetics. Instead of filling in each cell by hand, a calculator automates the process and shows the result instantly.

This page focuses on a single-gene cross with two alleles per parent. That means each parent contributes one allele to each offspring, and the four-cell Punnett square shows all possible combinations.

How to Use This Punnett Tool

1) Enter each parent genotype

Type two letters for each parent. Use uppercase for dominant alleles and lowercase for recessive alleles. Examples:

• AA (homozygous dominant)
• Aa (heterozygous)
• aa (homozygous recessive)

2) (Optional) Name the phenotypes

You can add plain-language labels like “Brown eyes” and “Blue eyes,” or leave those fields blank and use generic dominant/recessive output.

3) Click Calculate

The calculator builds the 2×2 Punnett square, counts genotype frequencies, and computes phenotype probabilities under complete dominance assumptions.

How to Read the Results

Punnett square grid

The top row and left column list each parent’s possible gametes. Every interior cell represents one possible offspring genotype.

Genotype distribution

You’ll see counts and percentages out of four outcomes, such as:

• AA: 1/4 (25%)
• Aa: 2/4 (50%)
• aa: 1/4 (25%)

Phenotype probability

If complete dominance applies, any offspring with at least one dominant allele shows the dominant phenotype. Only two recessive alleles produce the recessive phenotype.

Common Example Crosses

Aa × Aa

This is the classic monohybrid cross. Genotype ratio is typically 1:2:1, while phenotype ratio is 3:1 (dominant:recessive).

AA × aa

All offspring are heterozygous (Aa). Genotype outcome is 100% Aa, and phenotype is 100% dominant.

Aa × aa

Half the offspring are Aa, half are aa. Phenotype probability is usually 50% dominant and 50% recessive.

Important Limits to Remember

This calculator is intentionally straightforward and educational. Real inheritance can be more complex. Keep these limits in mind:

• Assumes one gene with two alleles
• Assumes complete dominance (not incomplete dominance or codominance)
• Does not model linked genes or epistasis
• Does not account for environmental effects on phenotype expression

Quick Genetics Glossary

• Allele: Different version of a gene (e.g., A or a)
• Genotype: Allele combination an organism carries (e.g., Aa)
• Phenotype: Observable trait expression (e.g., purple flower)
• Dominant: Expressed when at least one dominant allele is present
• Recessive: Expressed only when both alleles are recessive

Why This Calculator Is Useful

Whether you are reviewing for a biology test, teaching introductory genetics, or checking your own hand calculations, a Punnett calculator saves time and reduces arithmetic mistakes. It also helps learners connect symbolic genotype notation with real probability outcomes.