In genetics, the dash symbol (–) is a "wild card" that stands for either the dominant allele or the recessive allele; for example, A– means the individual has either the genotype AA or Aa. Two genes that undergo independent assortment affect flower color in sweet peas. Each gene as two alleles, one of which is dominant for flower color. Genotypes of the form A– B– have purple flowers, whereas those of the form A– bb, aa B–, and aa bb have white flowers. What ratio of purple : white is expected from the cross Aa Bb × Aa Bb?
Hint: This type of epistasis is known as "duplicate recessive epistasis."
A. 9:7
B. 10:6
C. 12:4
D. 13:3
E. 15:1