Npea plants there is complete dominance for the trait of flower color. the p allele produces purple flowers and p produces white flowers. a cross of two pea plants which are both heterozygous for flower color will produce offspring with the genotypic ratio

PP-  purple homozygote
pp- white homozygote
Pp- purple hetrozygote

If these flowers cross, we obtain heterozygote offsprings with a genotype: Pp (100% of them will be like this)

Now, if those offsprings cross with each other: Pp x Pp
results:
  1/4 would be purple  homozygote -> PP
  1/4 would be white -> pp
  2/4 would be purple heterozygotes -> Pp
Genotipic ratio: 3:1 (3 are purple; 1 is white)