Reginald Punnett created the Punnett square, a diagram used to predict possible offspring genotypes and phenotypes from a genetic cross. A Punnett square works by writing the alleles of one parent across the top and the other parent down the side, then determining the genotypes and phenotypes of the offspring through combination. An example problem shows a black heterozygous female guinea pig mating with a white male, resulting in offspring with 25% BB, 50% Bb, and 25% bb genotypes, and 75% black and 25% white phenotypes.