Punnett Square Calculator
This Punnett square calculator crosses two parent genotypes and instantly builds the full offspring grid, along with the resulting genotype and phenotype ratios, for both monohybrid (one gene) and dihybrid (two gene) crosses.
A Punnett square represents expected offspring genotype probabilities by listing the gametes each parent can produce and pairing them in a grid. This calculator handles one- and two-gene crosses under a simple Mendelian model: two alleles per gene, complete dominance for phenotype labels, and independent assortment for the two-gene mode.
unique gametes(P1) × unique gametes(P2) = offspring gridStep-by-step breakdown
- Enter both parent genotypes above to see the gamete logic unfold step by step.
Use a Punnett Square to Model Simple Mendelian Crosses
A Punnett square is a probability model. It lists possible parental gametes and combines them to show the expected frequencies of offspring genotypes. For a heterozygous monohybrid cross such as Aa × Aa, equal segregation gives the familiar 1:2:1 genotype ratio and, under complete dominance, a 3:1 phenotype ratio.
In the two-gene mode, this calculator generates one allele from each gene for every gamete and treats those genes as assorting independently. For AaBb × AaBb, that produces four gamete types per parent and 16 grid cells. Under complete dominance and independent assortment, the expected phenotype ratio is 9:3:3:1.
Those assumptions matter. Genes that are linked can depart from independent-assortment expectations, and phenotype categories can differ when inheritance involves codominance, incomplete dominance, epistasis, or other mechanisms. Use this tool for the simple model it states rather than as a universal genetics predictor.
How This Punnett Square Generator Builds the Grid
Each parent’s genotype splits into possible gametes, the allele combinations that could end up in a sperm or egg cell. The calculator lines up one parent’s gametes across the top and the other’s down the side, then fills in every possible offspring combination.
Reading this calculator’s notation
For this simple model, uppercase and lowercase forms of the same letter represent the two alleles of one gene; uppercase is treated as dominant for phenotype grouping. Aa is heterozygous, while AA and aa are homozygous.
Monohybrid vs. dihybrid
A monohybrid cross tracks one gene (e.g. Aa × Aa, a 2×2 grid). A dihybrid cross tracks two genes at once (e.g. AaBb × AaBb, a 4×4 grid), assuming the genes assort independently.
Genotype vs. modeled phenotype
Genotype is the allele combination, such as Aa. In this calculator’s complete-dominance model, any genotype containing the uppercase allele is grouped into the dominant phenotype category.
Where the classic 3:1 comes from
Crossing two heterozygotes for one gene (Aa × Aa) produces genotypes in a 1:2:1 ratio (AA:Aa:aa), which collapses to a 3:1 phenotype ratio since both AA and Aa show the dominant trait.
Punnett Square Calculator Examples
These examples use the same simple Mendelian assumptions as the calculator: equal segregation, complete dominance for phenotype grouping, and independent assortment in the two-gene example.
Aa × Aa
Under complete dominance, AA and Aa are grouped together as the dominant phenotype.
Aa × aa
This expected ratio follows from the simple complete-dominance model; observed offspring can vary by chance.
AaBb × AaBb
This classic ratio requires independent assortment and complete dominance at both genes.
Punnett Square Mistakes to Avoid
Mixing up which letter is dominant
In this calculator’s notation, uppercase and lowercase forms of the same letter represent the two alleles of one gene. A monohybrid entry such as Ab is rejected because the letters do not identify the same modeled gene.
Forgetting genotype letter order for dihybrid crosses
A dihybrid genotype like AaBb must list both alleles of gene A first, then both alleles of gene B. Entering the letters out of order changes which trait each pair represents.
Assuming independent assortment always applies
The dihybrid mode assumes independent assortment. Genes on different, non-homologous chromosomes generally follow the independent-assortment model used here. Genes close together on the same chromosome can be linked, while recombination can make genes farther apart on the same chromosome behave more like independently assorting genes.
Treating codominant or incomplete-dominance traits like simple dominance
This calculator assumes one allele fully masks the other. Traits involving codominance (such as ABO blood-group alleles) or incomplete dominance follow different phenotype rules and won’t match this calculator’s complete-dominance output.
Confusing genotype ratio with phenotype ratio
A monohybrid Aa × Aa cross gives a 1:2:1 genotype ratio but only a 3:1 phenotype ratio, since both AA and Aa look the same outwardly. Quoting the wrong one changes the interpretation of a result.
Punnett Square Calculator: Frequently Asked Questions
How do I enter a genotype into this Punnett square calculator?+
Aa. For a dihybrid cross, use four letters covering two genes back-to-back, like AaBb, where the first pair is gene A and the second pair is gene B.What do uppercase and lowercase letters mean?+
Why does the calculator assume independent assortment?+
Does this handle codominance or incomplete dominance?+
Can I cross more than two genes?+
Why do phenotype and genotype ratio totals differ?+
About This Calculator
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Punnett Square Calculator References
These references support the Mendelian segregation, independent-assortment, linkage, and classic ratio concepts used or discussed on this page.