How a Punnett square works

Each parent passes one allele per gene to each offspring, so the first step is to list the possible gametes — every combination of one allele from each gene. A heterozygote for two genes (AaBb) makes four gametes: AB, Ab, aB, ab. Put one parent's gametes across the top and the other's down the side, then fill each box by combining that row and column. Counting the boxes gives the genotype ratio; grouping boxes that show the same trait (dominant if at least one capital allele is present for that gene) gives the phenotype ratio — which is where the classic 3:1 monohybrid and 9:3:3:1 dihybrid ratios come from.

Assumes complete dominance and independent assortment; uppercase letters are dominant alleles, lowercase recessive. Works for one or two genes.

Related tools: Hardy-Weinberg calculator · all biochem tools.