Punnett Square Calculator
Build a single-gene Punnett square in seconds. Give each parent two alleles — the default is dominant A and recessive a, but you can type any letters you like. We fill the 2×2 grid, sort each genotype dominant-first, and work out the genotype and phenotype ratios for you. It is a friendly genetics study tool for biology homework and curious minds, with the science kept honest: textbook crosses are a clean model, while real human traits are far messier than one square suggests.
Punnett square
- Genotype ratio
- 1 AA : 2 Aa : 1 aa
- Phenotype ratio
- 3 dominant : 1 recessive
Uppercase = dominant allele, lowercase = recessive. Any combination with at least one uppercase letter shows the dominant trait. Real human traits are usually shaped by many genes, so a single square is a teaching model, not a prediction.
How a Punnett square works
A Punnett square is a simple grid that crosses two parents for one gene. Each parent carries two alleles — versions of that gene. Put one parent’s alleles along the top as columns and the other parent’s alleles down the side as rows, then combine them cell by cell. The four inner cells list every allele pair an offspring could inherit, and because each cell is equally likely, the square doubles as a probability map.
By convention an uppercase letter is the dominant allele and the matching lowercase letter is recessive. A genotype shows the dominant trait if it carries at least one uppercase allele, so both AA and Aa look dominant, and only aa shows the recessive trait. That single rule is what turns a genotype ratio into a phenotype ratio.
Reading the ratios
- Aa × Aa → genotype 1 AA : 2 Aa : 1 aa, phenotype 3 dominant : 1 recessive. The famous monohybrid 3:1.
- AA × aa → every cell is Aa, so all offspring are carriers and all show the dominant trait.
- Aa × aa → a 1 : 1 split between Aa and aa, meaning half show the dominant trait and half the recessive — a classic test cross.
- aa × aa → every cell is aa, so all offspring show the recessive trait.
These ratios describe probabilities across many offspring, not a guarantee for any single one. Flipping four heads in a row is unlikely but possible, and genetics works the same way.
The honest part: most traits are not this simple
The clean dominant-recessive square is a brilliant teaching tool, and it is exactly right for a small number of single-gene patterns. But the old textbook idea that everyday human features — earlobe attachment, tongue rolling, a widow’s peak, dimples, or the bend of a hitchhiker’s thumb — are simple one-gene dominant-recessive traits is now known to be a myth. Modern genetics shows these are polygenic: shaped by many genes acting together, often with environmental influence on top. You can still draw a Punnett square for them as a learning exercise, but it will not actually predict the trait in a real family.
So use this calculator the way it shines — to understand inheritance, dominance, and probability — and treat any “baby trait” guesses as fun and educational, never as a scientific or medical prediction.
Frequently asked questions
- How do you make a Punnett square?
- Write one parent's two alleles across the top of a 2×2 grid and the other parent's two alleles down the left side. Fill each inner cell by combining the allele from its column with the allele from its row. The four cells show every genotype an offspring could inherit, each with equal probability. This calculator builds that grid for you the moment you type the four alleles.
- What is the difference between a genotype and a phenotype?
- The genotype is the exact pair of alleles an individual carries, such as AA, Aa, or aa. The phenotype is the observable trait those alleles produce. With simple dominance, both AA and Aa show the dominant phenotype because one dominant allele is enough, while only aa shows the recessive phenotype. That is why a 1 AA : 2 Aa : 1 aa genotype ratio collapses to a 3 dominant : 1 recessive phenotype ratio.
- Why does Aa × Aa give a 3:1 ratio?
- Crossing two heterozygous parents (Aa × Aa) produces four equally likely cells: AA, Aa, Aa, and aa. Three of those four carry at least one dominant A, so three show the dominant trait and one shows the recessive trait — a 3:1 phenotype ratio, with a 1:2:1 genotype ratio underneath it. This is the classic monohybrid cross Gregor Mendel first described.
- Can a Punnett square predict my baby's traits?
- Not reliably. A Punnett square is a clean teaching model for a single gene with simple dominance. Almost every visible human trait — height, skin tone, eye colour, hair type — is polygenic, meaning many genes plus the environment shape it together. Treat the square as an educational illustration of probability, not a prediction of how a real child will look.
- Does this calculator save my inputs?
- No. Everything runs in your browser. Nothing is sent to a server, stored, or shared, and refreshing the page resets the alleles. It is a private, instant genetics tool with no account or sign-up.
Related tools
- Genetic Traits Explorer — which traits really follow simple inheritance, and which are myths
- Baby Eye Color Calculator — a simplified, just-for-fun estimate of a child’s eye colour
- Blood Type Calculator — a real-world inheritance example with multiple alleles
- Child Height Predictor — a polygenic trait shaped by both parents and environment
— The Period Tools Team