Final answer:
The letters on the outside of a Punnett square represent the parents' genotypes. The Punnett square is used to predict the possible genotypes and phenotypic ratios of offspring from two parents.
Step-by-step explanation:
A Punnett square is a diagrammatic tool used in genetics to predict probable offspring genotypes from a cross between two parent organisms. The letters on the outside of a Punnett square stand for the parents' genotypes, while the interior of the grid shows the possible genetic combinations of their offspring.
For example, if we consider a monohybrid cross of true-breeding pea plants with yellow seeds (YY) and green seeds (yy), the Punnett square would list YY along one side and yy along the other, indicating the parents' genotypes. As these parents produce gametes, their alleles segregate, and the square visually represents the combination of these gametes - each interior box showing a potential genotype of the resulting zygote.
This method, developed by British geneticist Reginald Punnett, allows for easy determination of expected genotypic and phenotypic ratios among offspring.