EPISTASIS

III. EPISTASIS - when one gene affects the expression of another.

Suppose you have the following cross:

P   True-breeding White flowered Pea Plant............................True-breeding White
..............Strain #1.......................................................X...............Strain #2

F1   All Purple Flowered

F2   Purple:White
...........9:7

How is this possible? 

2 Genes are controlling this trait:
A = Purple allele
a = White allele
B = Allows Purple
b = Doesn't allow Purple

P   AAbb.........................X..................aaBB

F1   All AaBb

F2 will yield 9 that are A_B_ (purple) and 7 that are either aa_ _, or _ _ bb (white)

This is caused by the following enzymatic pathway:

Colorless Chemical #1 -----------------------> Colorless Chemical #2 --------------> Purple Pigment
..........................................Enzyme B.....................................................Enzyme A