Type two parent genotypes such as AaBb × AaBb and this free Punnett square calculator lists each parent’s gametes, draws the full grid and gives the offspring genotype and phenotype ratios with percentages. Works for monohybrid, dihybrid and trihybrid crosses, with complete or incomplete dominance.
How a Punnett square works
Each parent passes on one allele of every gene in each gamete (Mendel’s law of segregation), and genes on different chromosomes sort independently (law of independent assortment). The square lists one parent’s possible gametes across the top and the other’s down the side; each box is one equally likely offspring.
Genotype vs phenotype
The genotype is the allele combination (AA, Aa or aa). The phenotype is the visible trait. With complete dominance, AA and Aa look the same, so they share a phenotype (written A_). With incomplete dominance, the heterozygote is intermediate (red × white → pink snapdragons), so every genotype is its own phenotype and the ratio is 1 : 2 : 1.
This calculator assumes unlinked autosomal genes. Linked genes, sex-linked traits and epistasis change the ratios.
Frequently asked questions
How do I use the Punnett square calculator?
Enter both parents’ genotypes using the same letters, for example AaBb and Aabb, and press the button. You’ll see each parent’s gametes, the full Punnett square and the genotype and phenotype ratios.
What is the 9:3:3:1 ratio?
It is the phenotype ratio from crossing two dihybrids (AaBb × AaBb) with complete dominance: 9 show both dominant traits, 3 and 3 show one dominant trait each, and 1 shows both recessive traits.
What is the probability of a recessive offspring?
From Aa × Aa, the chance of aa is 1 in 4 (25%). The results table lists the probability of every genotype and phenotype as a fraction and percentage.
Can it do a trihybrid cross?
Yes. Enter three genes per parent, such as AaBbCc. The square has 8 × 8 = 64 boxes and the classic phenotype ratio is 27:9:9:9:3:3:3:1.
What about incomplete dominance?
Choose Incomplete / codominance. Heterozygotes are then counted as their own phenotype, giving 1:2:1 instead of 3:1 for a monohybrid cross.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.