Punnett Square Calculator
Enter two parent genotypes. Get the grid and offspring ratios.
Enter each parent's genotype, for example AaBb and AaBb. The calculator lists every gamete, fills the Punnett square and counts the offspring genotypes and phenotypes. A dihybrid cross of two heterozygotes gives a 9:3:3:1 phenotype ratio and a monohybrid Aa × Aa cross gives 3:1.
Free, runs in your browser. Nothing you enter is uploaded.
What you enter
- Parent 1 genotype (e.g. AaBb)
- Parent 2 genotype with the same genes
What you get
- Gametes and the full Punnett square
- Genotype counts, ratio and percentages
- Phenotype counts, ratio and percentages
Formula
Gametes per parent = 2ⁿ (n genes) | Offspring fraction = cells with that genotype ÷ total cells
Each parent passes one allele of each gene to a gamete. With independent assortment every combination of alleles is equally likely, so each cell of the square has the same probability. Phenotypes assume complete dominance: any uppercase allele shows the dominant trait.
Questions
How do you do a Punnett square?
Write the gametes one parent can make along the top and the other parent's gametes down the side. Fill each cell by combining the row and column alleles. Count how often each genotype appears and divide by the number of cells to get its probability.
What is the ratio for a dihybrid cross?
AaBb × AaBb gives 16 cells. The phenotype ratio is 9:3:3:1 and the genotype ratio is 1:2:1:2:4:2:1:2:1, covering 9 genotypes.
What is the phenotype ratio of a monohybrid cross?
Aa × Aa gives 3 dominant to 1 recessive for the phenotype, and 1 AA : 2 Aa : 1 aa for the genotype. Aa × aa, a test cross, gives 1:1.
What does A_ mean in the phenotype column?
The underscore means either allele. A_ covers AA and Aa, which look the same when A is completely dominant.
Does this work for incomplete dominance or linked genes?
The genotype ratios are correct for any unlinked genes. The phenotype ratios assume complete dominance, so for incomplete dominance or codominance read the genotype ratios instead. Linked genes do not assort independently and need recombination frequencies.
References
- Griffiths AJF, Doebley J, Peichel C, Wassarman DA (2020). Introduction to Genetic Analysis, 12th ed. W.H. Freeman.
- Punnett RC (1905). Mendelism. Macmillan and Bowes, Cambridge.
More Genetics
All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.