A Punnett square is a checkerboard representation of predictable genotypes from a second generation offspring of an experimental breeding. Named for geneticist Reginald C. Punnett, who originally used the method to compute the results of a cross using only one gene. Punnett squares are now occasionally used for considering two genes and their alleles, however they quickly become unwieldy when used to calculate offspring of more than two alleles.

With a Punnett square, one gene is considered and the genotype and phenotype are predicted. Along the top of the checkerboard are the alleles found in the gametes of one parent (usually the male by convention) and the alleles found in the other parent are written down the left-hand side. The products of the possible matings are then placed in the four boxes in the middle of the checkerboard. The relative numbers of the different progenies can...