Reginald Punnett, an English geneticist, developed the Punnett square to determine the potential genetic outcomes from a cross. Merriam-Webster says its first known use occurred in 1942. Heterozygous plants have a dominant and a recessive allele (alternate form) for a given trait. The Punnett square shows the genotype of each plant in a test cross along either side of the square. It also demonstrates each intersection between these genotypes, which results in a potential genetic outcome of their pairing.
Make the Punnett Square
Imagine you are crossing two heterozygous plants in which curly leaves, which are C, and rough leaves, which are R, are dominant. Flat leaves, which are c, are recessive. Smooth leaves, which are r, are also recessive.
Four possible combinations of these alleles exist. These are CR, Cr, cR, and cr. To determine all possible outcomes from the pairings of these genotypic combinations, do the following:
Draw a square and then subdivide the square four squares. Then, subdivide each of the four squares into four smaller squares. You now should have 16 small squares within the original, larger square.
Then, on the left-hand side of your now-four smaller squares, list one of each of these potential genotypes in order such that they correspond with the outer left edge of one of the newly created squares as follows: CR, Cr, cR, and cr.
Then, do the same above the upper edge of the overall square by listing:CR, Cr, cR, and cr. You can change the order of these genotypes, but you should ensure that your genotypic options are listed in the same order on both the top and the left of your square.
So, for example, if you begin your list on the left edge with cr you will need to begin with the same genotype on the upper edge.
Calculate the Potential Outcomes
Create new genotypic pairs with your square, following the intersections of the alleles in your Punnett square. For example, at the intersection between CR from the left and CR from above, write CRCR.
Continue recording these potential outcomes across each of the 16 squares. The end result is a diagram that reflects the potential outcomes of this genetic cross, which can help you propose probabilities of particular genotypes.