Apea plant with purple flowers (and unknown genotype) is crossed with a pea plant with white flowers. one-half of the offspring have purple flowers and one-half have white flowers. what are the genotypes of the parent plants? see section 11.3 (page 177) .

The genotypes of the parents are Aa (heterozygous) and aa (recessive homozygous).  
 For example:  
 If the purple flower is dominant phenotype, then A is a dominant allele for it and Aa is a genotype which will give the purple colour.
In this case, white flower is recessive phenotype with aa genotype.
If we cross Aa x aa
The offspring is going to be Aa Aa aa aa (half purple and half white)