By Teresa J. Siskin, Updated Aug 30, 2022
Reginald Punnett, an English geneticist, introduced the Punnett square in 1942 as a tool to predict the genetic outcomes of a cross. For heterozygous plants—those carrying one dominant and one recessive allele—this diagram shows every possible genotype of their progeny.
Draw a large square and divide it into four equal quadrants. Then, subdivide each quadrant into four smaller squares. The result is a 4 × 4 grid containing 16 cells.
At the top edge of the grid, list the four possible genotypes of one parent: CR, Cr, cR, cr. Repeat the same order along the left edge for the second parent. The sequence must match on both sides so that each cell’s intersection corresponds to the pairing of those two genotypes.
For each of the 16 cells, combine the alleles from the intersecting genotypes. For example, the cell where the top row’s CR meets the left column’s CR receives the genotype CRCR. Continue this process across the entire grid.
Each filled cell represents a potential genotype of an offspring. By counting the occurrences of each genotype, you can calculate the probability of each outcome, providing a clear picture of the genetic distribution in the progeny.
