About this tool
Enter the two parent genotypes (for example Aa or AaBb) and this tool builds the full Punnett square, then tallies every offspring genotype and phenotype. It supports one-gene (monohybrid) and two-gene (dihybrid) crosses. Everything runs in your browser — nothing is uploaded.
Each genotype is split into allele pairs by gene. The gametes are the Cartesian product of picking one allele from each gene, so a heterozygous single gene gives two gametes and a double heterozygote gives four. Every gamete of parent one is combined with every gamete of parent two to fill the grid; each offspring gene pair is written with the uppercase (dominant) allele first. Genotypes are then counted and the ratio is reduced by the greatest common divisor. A gene shows the dominant phenotype whenever at least one uppercase allele is present, which is how the phenotype ratio is built.
A classic monohybrid cross Aa × Aa yields a 1:2:1 genotype ratio (AA:Aa:aa) and a 3:1 phenotype ratio, while the dihybrid AaBb × AaBb gives the famous 9:3:3:1 phenotype ratio. Use uppercase letters for dominant alleles and lowercase for recessive, and keep the same gene letters in both parents.