Punnett Squares:
A Punnett square is used to organize the possible combinations of offspring from particular parents. The alleles for a true-breeding, purple-flowered plant are written as PP. The alleles for true-breeding, white-flowered plant are written as pp. All the offspring have the same genotype: Pp. The dominant allele, P, in each genotype ensures that all of the offspring will be purple-flowered plants.