Fruit Flies Normally Have Red Eyes -Based on These Crosses, How Many Different Eye Color Genes

Multiple Choice

Fruit flies normally have red eyes.Seven different true-breeding strains of fly with white eyes have been identified (A-H) .In each strain, the white eye trait is due to an autosomal recessive allele.It is possible that all seven strains have mutations in the same gene.Alternatively, some or all of the strains may have mutations in different genes.To determine how many genes are involved in eye color in these flies, all possible pairwise crosses are performed.The offspring phenotypes resulting from each cross are observed.(+ = wild-type; ? = mutant)
$$\begin{array} { | c | c | c | c | c | c | c | c | c | } \hline & \mathrm { A } & \mathrm { B } & \mathrm { C } & \mathrm { D } & \mathrm { E } & \mathrm { F } & \mathrm { G } & \mathrm { H } \\\hline \mathrm { A } & - & - & + & + & - & + & + & - \\\hline \mathrm { B } & & - & + & + & - & + & + & - \\\hline \mathrm { C } & & & - & - & + & + & + & + \\\hline \mathrm { D } & & & & - & + & + & + & + \\\hline \mathrm { E } & & & & & - & + & + & - \\\hline \mathrm { F } & & & & & & - & - & + \\\hline \mathrm { G } & & & & & & & - & + \\\hline \mathrm { H } & & & & & & & & - \\\hline\end{array}$$
-Based on these crosses, how many different eye color genes are mutant in all of these strains collectively?

A) 1
B) 2
C) 3
D) 4
E) 8