Punnett Square Calculator
Enter each parent's two alleles for one gene to see the offspring genotype and phenotype ratios.
Use the same letter for both parents; capital = dominant, lowercase = recessive (e.g. Aa).
A Punnett square predicts offspring ratios from a genetic cross. For one gene, each parent passes one of their two alleles, giving four equally likely combinations. Crossing two heterozygotes (Aa × Aa) yields genotypes in a 1 AA : 2 Aa : 1 aa ratio, and a 3 : 1 dominant-to-recessive phenotype ratio.
How to Use This Calculator
Enter each parent's two alleles for a single gene, using the same letter for both parents — a capital letter for the dominant allele and lowercase for the recessive, such as Aa. The calculator crosses them and returns the genotype ratio among the four possible offspring and the resulting dominant-to-recessive phenotype odds.
This is a monohybrid cross — one gene at a time — the most common Punnett square used in introductory genetics.
How the Calculation Works
Each parent randomly passes one of its two alleles to the offspring. Pairing each of the first parent's alleles with each of the second's produces four equally likely combinations — the four cells of the Punnett square.
The genotype ratio counts how many of those four are homozygous dominant, heterozygous, and homozygous recessive. The phenotype depends on dominance: any offspring with at least one dominant allele shows the dominant trait, so only the homozygous-recessive cells show the recessive trait.
cross each of parent 1's alleles with each of parent 2's → 4 combinations
Worked Example
Cross Aa × Aa. The four combinations are AA, Aa, aA and aa, which group as 1 AA : 2 Aa : 1 aa — the classic 1 : 2 : 1 genotype ratio.
For phenotype, the three offspring with at least one A show the dominant trait and only aa shows the recessive, giving a 3 : 1 ratio, or 75% dominant and 25% recessive.
Assumptions and Limitations
- Handles a single-gene (monohybrid) cross with simple dominance
- Assumes complete dominance — not codominance or incomplete dominance
- Each allele is passed independently and with equal probability
- Real inheritance can involve many genes and other complications
Who Uses a Punnett Square Calculator
Biology and genetics students, teachers, and hobby breeders use this to cross two parents and see the offspring genotype and phenotype ratios. Run it for genetics homework, to plan or predict a breeding outcome, or to understand how a single trait is inherited. Enter the parents' genotypes to get the full ratio.
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Open calculator →Frequently Asked Questions
How does a Punnett square work?
It pairs each allele one parent can pass with each allele the other can pass, giving four equally likely offspring combinations. From those you read the genotype and phenotype ratios.
Can two brown-eyed (Aa) parents have a recessive-trait child?
Yes. Crossing Aa × Aa gives a 1-in-4 (25%) chance of an aa child showing the recessive trait, even though both parents show the dominant trait.
What is the difference between genotype and phenotype?
Genotype is the pair of alleles (like Aa); phenotype is the visible trait they produce. With complete dominance, AA and Aa share the same phenotype.
What is a 3:1 ratio?
It is the phenotype ratio from crossing two heterozygotes (Aa × Aa): three offspring show the dominant trait for every one showing the recessive, i.e. 75% to 25%.
Does this handle two genes?
No, this is a single-gene cross. A dihybrid (two-gene) cross uses a 4×4 Punnett square and produces the classic 9:3:3:1 ratio.
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