Final answer:
The white flower phenotype is expressed by the genotype pp in pea plants. For Punnett square analysis, a 2x2 square is used for a cross between PpYY and ppyy, producing genotypes with purple flowers and yellow peas except for the recessive pp genotype resulting in white flowers.
Step-by-step explanation:
The genotype that would have a phenotype of a white flower in pea plants is pp. In the context of genetics, purple flowers (P) are dominant over white flowers (p), therefore the presence of at least one P allele will result in a purple flower. Only when both alleles are recessive (pp) does the white flower phenotype manifest. For a cross between a PpYY and ppYy pea plant, there are different genotypes depending on the combination of genes, but for flower color specifically, the possible genotypes would be Pp and pp, and only the pp genotype would result in white flowers.
The Punnett square analysis of a cross between PpYY and ppyy pea plants will yield genotypes PpYY, PpYy, ppYY, ppYy, all having yellow peas (since Y is dominant), with the first three having purple flowers and the last one having white flowers. For the cross between PpYY and ppyy, you would need a 2x2 Punnett square, while a cross between PpYY and ppYy would require a slightly larger Punnett square due to the varying alleles.