Final answer:
The sides of a Punnett square show the gametes from each parent, which combine to predict possible genotypes of the offspring.
Step-by-step explanation:
The elements listed along the top and side of a Punnett square represent the gametes produced by each parent through meiotic segregation. During this process, diploid cells produce haploid gametes, one allele for each gene. In a monohybrid cross using a Punnett square, if we consider parents with genotypes YY (yellow seed) and yy (green seed) respectively, we receive specific combinations of these alleles when fertilization occurs.
Each parent contributes one allele to the offspring, resulting in genotypes like YY, Yy, or yy. For the case of reciprocal crosses, heterozygous offspring producing gametes Y and y can create a variety of offspring genotypes. The resulting Punnett square showcases the potential genetic makeup of the zygote, predicting a phenotypic ratio according to Mendelian inheritance patterns.