Punnett square calculator.
Work out a monohybrid cross. Choose each parent’s genotype for one gene and the calculator gives the offspring genotype ratio and the dominant to recessive phenotype split.
Parent genotypes
LiveA uppercase is the dominant allele, a lowercase the recessive. The cross gives offspring as AA : Aa : aa out of four.
Genotype ratio
1 : 2 : 1
AA : Aa : aa
A single-gene (monohybrid) cross with simple dominance. Calculations run in your browser; nothing you enter is stored.
How it works
One gene, two alleles
A Punnett square predicts the offspring of a cross by combining one allele from each parent in every possible way. For a single gene with a dominant allele A and recessive a, each parent contributes one of their two alleles, giving four equally likely combinations.
Crossing two heterozygotes, Aa by Aa, gives one AA, two Aa and one aa, a genotype ratio of 1 to 2 to 1. Because A is dominant, three of the four show the dominant trait, a 3 to 1 phenotype ratio.
Reference
Common monohybrid crosses.
The offspring genotypes for each pairing of single-gene parent genotypes.
| Cross | Offspring |
|---|---|
| AA x AA | All AA |
| AA x aa | All Aa |
| Aa x Aa | 1 : 2 : 1 |
| Aa x aa | 1 Aa : 1 aa |
| aa x aa | All aa |
The full guide
The complete guide to Punnett squares.
What a Punnett square shows, how to read genotype and phenotype ratios, and its limits.
What a Punnett square shows
A Punnett square is a grid that lays out every way two parents’ alleles can combine in their offspring. For one gene, each parent has two alleles and passes one at random, so the square has four cells, each an equally likely genotype.
It turns inheritance into simple probabilities, which is why it is the first tool taught in genetics.
Genotype versus phenotype
The genotype is the allele pair an individual carries, like AA, Aa or aa. The phenotype is the trait you see, which depends on dominance: with a dominant A, both AA and Aa look dominant, while only aa shows the recessive trait.
So an Aa by Aa cross gives a 1:2:1 genotype ratio but a 3:1 phenotype ratio.
Reading the ratios
The genotype ratio counts the four offspring cells by type. The phenotype ratio groups them by appearance. Percentages follow directly: in a 3:1 split, 75% show the dominant trait and 25% the recessive.
These are expected ratios over many offspring, not a guarantee for any single one.
Limits of the model
This calculator covers a monohybrid cross with simple dominance. Real inheritance can involve codominance, incomplete dominance, multiple genes, linkage and sex linkage, which need more complex tools like a dihybrid square.
Use it for single-gene problems, and step up to a dihybrid or trihybrid cross for two or three genes.
The formula
Combine the
alleles.
Each parent passes one allele; the four combinations give the genotype ratio, then dominance gives the phenotype.
Dihybrid cross ›# Monohybrid cross
each parent passes one allele
4 equally likely combinations
# Aa x Aa
AA : Aa : aa = 1 : 2 : 1 (3:1 phenotype)Questions
Punnett questions.
How do I use a Punnett square?
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Take the two alleles from each parent and combine them in all four ways. Choose both parent genotypes above and the calculator gives the offspring genotype and phenotype ratios.
What is the result of an Aa x Aa cross?
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A genotype ratio of 1 AA : 2 Aa : 1 aa, and a 3 to 1 phenotype ratio, so 75% show the dominant trait and 25% the recessive.
What is the difference between genotype and phenotype ratio?
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Genotype ratio counts the allele pairs (AA, Aa, aa). Phenotype ratio groups them by visible trait, so AA and Aa both count as dominant.
Does this handle two genes?
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No, this is a single-gene monohybrid cross. For two or three genes, use the dihybrid or trihybrid cross calculators.
Is this Punnett square calculator free?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The Punnett square calculator covers single-gene crosses with simple dominance, for education.