🧬Punnett Square Calculator

About this tool

Type each parent's genotype as letter pairs, one pair per gene, with capitals for dominant alleles, such as Aa × Aa or AaBb × AaBb. The calculator lists each parent's gametes, draws the Punnett square and gives the phenotype and genotype ratios with fractions and percentages. Up to three genes are supported.

How Punnett squares work

A Punnett square, devised by Reginald Punnett in the early 1900s, shows every way that two parents' gametes can combine. Each parent passes one allele of each gene to a gamete, so the gametes go along the top and side and each box holds one possible offspring genotype. In a cross between two heterozygotes, Aa × Aa, the boxes are AA, Aa, Aa and aa: a 1 : 2 : 1 genotype ratio. If A is dominant, AA and Aa look the same, giving the classic 3 : 1 ratio of dominant to recessive appearance that Gregor Mendel found in pea plants. With two genes on different chromosomes, each parent makes four kinds of gamete, the square has 16 boxes, and a dihybrid cross AaBb × AaBb gives the famous 9 : 3 : 3 : 1 phenotype ratio. A cross with a homozygous recessive parent, called a test cross, reveals whether the other parent is heterozygous. In the phenotype list, A_ means at least one dominant allele. Honest limits: Punnett squares assume simple Mendelian inheritance, with complete dominance and genes that sort independently. They don't model incomplete dominance, co-dominance, sex-linked or linked genes, or traits shaped by many genes, and the ratios are probabilities rather than guarantees for any one family. The blood type calculator and the DNA translator are related.

Frequently asked questions