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

Option A, 9:7

Explanation:

Given,

Genotype of A-B- have purple color

A-B-  have genotype AABB, AaBb, AaBB, AABb

While genotype of A– bb, aa B–, and aa bb have white flowers

A– bb have genotype - AAbb, Aabb

aa B– have genotype aaBb, aaBB

Cross between Aa Bb and AaBb will give following offspring

AB         Ab         aB          ab

ABAABBAABbAaBBAaBb

AbAABbAAbbAaBbAabb

aBAaBBAaBbaaBBaaBb

abAaBbAAbbaaBbaabb

So the possible genotypes and their corresponding phenotypes are  

AABB-1 (Purple)

AABb-1 (Purple)

AaBB-3 (Purple)

AaBb -4 (Purple)

AAbb-2 (White)

Aabb-1 (White)

aaBB-1 (White)

aaBb-2 (White)

aabb-1 (White)

Purple : White = 9:7

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.