How to Use a Punnett Square

Setting it up

1. Write the alleles each parent can pass on. An Aa parent gives A or a.
2. Put one parent's alleles along the top and the other's down the side.
3. Fill each box with the allele from its column and its row.

One gene: Aa × Aa

The four boxes are AA, Aa, Aa and aa, a genotype ratio of 1 : 2 : 1. If A is dominant, three of the four show the dominant trait, a phenotype ratio of 3 : 1. Each box is a probability for each child, not a promise about a family of four.

Two genes: AaBb × AaBb

Each parent can pass on AB, Ab, aB or ab, so the square is 4 × 4 with 16 boxes. When the genes sort independently and both show simple dominance, the phenotypes come out 9 : 3 : 3 : 1.

From families to populations

Hardy-Weinberg does the same for a whole population. If p and q are the frequencies of A and a, the genotypes are expected at p², 2pq and q². If 16% of people are aa, q = 0.4, p = 0.6 and 48% are carriers (Aa).

Calculators

The Punnett Square Calculator draws the square for any cross and gives the ratios. The Hardy-Weinberg Calculator gives allele frequencies from genotype counts and tests whether a population is in equilibrium.