Punnett Square Calculator
Cross two parent genotypes for a single gene and get the 2x2 Punnett square, genotype ratio, and phenotype ratio.
🌸 What is the Punnett Square Calculator?
The Punnett square calculator builds the classic 2x2 grid used to predict the possible genotypes and phenotypes of offspring from a monohybrid cross, a genetic cross involving a single gene with two alleles. Named after British geneticist Reginald Punnett, the Punnett square is the standard teaching tool for Mendelian inheritance, showing every possible combination of one allele from each parent and how often each combination is expected to occur.
Biology students use this calculator to check homework on genetics problems without manually drawing and filling in the grid every time. Teachers use it to quickly generate example crosses for different genotype combinations when building lesson materials. Anyone curious about basic trait inheritance, from pea flower color to simple pet coat color genetics, can use it to see exactly how a genotype ratio becomes a phenotype ratio.
A common point of confusion is mixing up genotype ratio and phenotype ratio. The genotype ratio (like 1:2:1 for Aa x Aa) counts every distinct allele combination separately, while the phenotype ratio (like 3:1 for the same cross) groups genotypes that produce the same observable trait, since AA and Aa both look dominant when A is fully dominant over a.
This calculator is useful because it instantly builds the correct grid for any of the nine possible parent genotype pairings (AA, Aa, or aa for each parent), reduces both ratios to their simplest whole-number form, and lets you rename the dominant and recessive traits so the output reads naturally for your specific scenario.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Heterozygous Cross (Aa × Aa)
Parent 1 = Aa (Purple flowers), Parent 2 = Aa (Purple flowers)
Example 2 — Test Cross (Aa × aa)
Parent 1 = Aa (Purple flowers), Parent 2 = aa (White flowers)
Example 3 — Homozygous Cross (AA × aa)
Parent 1 = AA (Purple flowers), Parent 2 = aa (White flowers)
❓ Frequently Asked Questions
🔗 Related Calculators
What is a Punnett square?
A Punnett square is a diagram used to predict the possible genotypes of offspring from a genetic cross. Each parent's alleles are placed along one row and one column of a grid, and each cell shows one possible offspring genotype, giving the expected genotype and phenotype ratios.
How do you make a Punnett square for a monohybrid cross?
Write each parent's two alleles for the gene along the top row and left column of a 2x2 grid. Fill each of the 4 cells by combining the allele from that cell's row with the allele from that cell's column, this gives every possible offspring genotype with equal probability.
Why does Aa x Aa give a 3:1 ratio?
Crossing two heterozygotes (Aa x Aa) produces offspring genotypes AA, Aa, Aa, and aa, one of each of the four equally likely combinations. Since AA and Aa both show the dominant phenotype, 3 of the 4 combinations (75%) show the dominant trait and 1 of 4 (25%) shows the recessive trait, a 3:1 ratio.
What genotype ratio results from Aa x aa (a test cross)?
Crossing a heterozygote with a homozygous recessive (Aa x aa) gives offspring genotypes Aa, Aa, aa, and aa, a 1:1 ratio of Aa to aa. This means the phenotype ratio is also 1:1, half the offspring show the dominant trait and half show the recessive trait.
What happens when you cross AA x aa?
Crossing a homozygous dominant parent (AA) with a homozygous recessive parent (aa) produces offspring that are all Aa, every offspring inherits one A allele and one a allele. All offspring show the dominant phenotype, a 4:0 (all-dominant) result with no genotype variation.
What is the difference between genotype and phenotype?
Genotype is the actual pair of alleles an organism carries for a gene, written like AA, Aa, or aa. Phenotype is the observable trait that genotype produces, since A is dominant, both AA and Aa produce the same dominant phenotype, while only aa produces the recessive phenotype.
Can a Punnett square predict the exact outcome for a specific offspring?
No. A Punnett square gives probabilities, not certainties, for any single offspring. A 3:1 ratio means each individual offspring has a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype, not that exactly 3 out of every 4 offspring will show it.
What is a test cross and why is it used?
A test cross breeds an individual showing the dominant phenotype (genotype unknown, either AA or Aa) with a homozygous recessive individual (aa). If any offspring show the recessive phenotype, the unknown parent must have been Aa, if all offspring show the dominant phenotype, the unknown parent was most likely AA.
Does this calculator work for any trait, not just Mendel's peas?
Yes. This calculator works for any single gene with two alleles showing simple complete dominance, you can rename the dominant and recessive trait labels to match your own scenario, whether it is flower color, seed shape, or a genetics homework problem involving fur color or blood type alleles.
What does a 1:2:1 genotype ratio mean?
A 1:2:1 genotype ratio (from Aa x Aa) means for every 4 offspring, on average 1 is homozygous dominant (AA), 2 are heterozygous (Aa), and 1 is homozygous recessive (aa). This is different from the 3:1 phenotype ratio, which groups AA and Aa together since they look the same.
Why do real offspring counts not always match the predicted ratio exactly?
Punnett square ratios are theoretical probabilities based on random assortment of alleles, actual small sample sizes (like a single litter of 4 puppies) are subject to chance and will often deviate from the exact predicted ratio, larger sample sizes tend to approach the predicted ratio more closely, per the law of large numbers.