The gene for flower color for pea plants codes for two types of flower color, purple and white flowers. You do a test cross between a purple and white flower. All out of the 18 flowers in the next generation are purple. What is the genotype of the purple and white flower in the P generation? What are the genotypes of the purple flowers in the F1 generation?

P generation - PP and pp

F1 generation - Pp

Explanation:

Firstly, a test cross is a cross performed between a an unknown dominant phenotype and a homozygous recessive trait in order to determine the genotype of the unknown phenotype. In this case, the flower colour gene is involved with two alleles: Purple allele and white allele. A test cross between a plant with dominant phenotype (purple flower, genotype unknown) and a homozygous recessive genotype (white flower, pp) results in 18 offsprings that are all purple flowered.

This means that the dominant parent with the unknown genotype is homozygous dominant i. e. PP, since it will produce only dominant alleles in meiosis that will later mask the expression of the recessive alleles produced by the second parent.

To the second question, the parents are: PP * pp. Hence, each parent will produce only P and p alleles in gametes respectively. Hence, the offsprings will have a Pp genotype.