What Is the Purpose of a Punnett Square?

A Punnett square is used to make predictions about the likelihood of certain traits occurring in offspring based on the genetics of the parents. This mathematical tool was invented by English geneticist Reginald Punnett in the early 20th century.

The simplest Punnett square uses a grid with four boxes in two columns and two rows. The genotype of one parent goes along the top of the grid and the genotype of the other parent goes along the left side. The genotypes contain two letters and represent the possible genes the parent could pass on.

As an example, if a gene for brown eyes is represented by "B" and a gene for green eyes is represented by "g", then there are three possible two-letter genotypes the parents can have: BB, Bg and gg. One parent has the "Bg" genotype, so these two letters go along the top two columns. The other parent has the "gg" genotype, so these two letters go along the left of the two rows.

Next, the empty squares are filled in by combining the letters at the head of each row and column. In the above example, the letters combine to form "Bg" and "gg" in the squares of the top row and "Bg" and "gg" in the bottom row.

These four filled-in squares now represent the probability an offspring has that genotype. In this example, there is a 50 percent chance this offspring has the "Bg" genotype and a 50 percent chance the offspring has the "gg" genotype.