When can two parent plants with purple flowers produce
offspring with white flowers?

if purple is the dominant allele (P) and white is the recessive allele (p) then it would be possible if both parents had the genotype Pp, then it would be possible for both parents to pass on the recessive gene (p) and have offspring with white flowers

Explanation:

children tend to take more after their grandparents more than their parents. it’s the dna you received that was passed down to you somewhere in your family. maybe one of your grandparents had/has straight hair and you might have gotten it from them.