Answer:
A) two genes are involved with 12:3:1 epistasis, such that A_B_ and A_bb are black, aaB_ is brown, and aabb is green .
Explanation:
The F2 proportions approximate 12:3:1 ratios:
• 239/320 ≅ 12/16
• 61/320 ≅ 3/16
• 20/320 = 1/16
These ratios are explained by dominant epistasis: if one gene has a dominant allele, then the expression of another gene will be masked, regardless of what alleles that second gene has.
The pure-breeding crosses were:
P Â AABB x aabb
F1 Â Â Â AaBb
F1 x F1 crosses
AaBb x AaBb
F2 (from Punnett Square):
9 A_B_ Â black
3 A_bb  black
3 aaB_ Â brown
1 aabb  green
12/16 individuals of the F2 are black, 3/16 are brown and 1/16 is green