Punnett square calculator.

Reading a Punnett square

A Punnett square lists the gametes each parent can pass on along its two edges and fills every box with the genotype that results when the two meet (OpenStax Biology 2e, 12.2). This tool assumes autosomal genes, complete dominance (an upper-case allele hides its lower-case partner) and, for two genes, independent assortment. The boxes are counted, so a cross gives exact fractions rather than estimates.

Worked example. Two heterozygous parents, Aa × Aa, give 1 AA : 2 Aa : 1 aa out of 4 boxes, a genotype ratio of 1:2:1 and a phenotype ratio of 3:1, so each child has a 75% chance of the dominant trait. A test cross uses a homozygous recessive partner (OpenStax 12.2): Aa × aa gives 2 Aa : 2 aa, a 1:1 phenotype ratio, while AA × aa gives only 4 Aa. For two double heterozygotes, AaBb × AaBb, the tool shows 16 boxes and a 9:3:3:1 phenotype ratio; Wikipedia notes that 9:3:3:1 is the expected result for unlinked genes and that any other ratio points to something else, such as lethal alleles, epistasis or linked genes.