Punnett square

A Punnett square is a simple grid for working out what a cross can produce, much like a multiplication table for inheritance. You list one parent's possible gametes along the top and the other parent's down the side, then fill each cell by combining the alleles from its row and column.

Each box represents one equally likely way the two gametes could meet at fertilization, so counting the boxes gives the expected genotype ratio among offspring. Translating those genotypes into visible traits then gives the phenotype ratio. A cross of two heterozygotes, for instance, fills a 2x2 square that yields the familiar 1:2:1 genotypes and 3:1 phenotypes.

The tool assumes the alleles segregate independently and that every combination is equally likely, which is exactly Mendel's idealised model. It predicts probabilities, not certainties: a 3:1 expectation is the long-run average, and any single small family can stray from it just as a few coin flips can. For many genes at once the grid grows unwieldy and a probability calculation is easier.