Three genes on different chromosomes are responsible for three enzymes that catalyze the same reaction in grapes:

colorless compound –> red compound
(a+, b+, c+)

The normal functioning of any one of these genes is sufficient to convert the colorless compound to the red compound. The abnormal functioning of these genes is designated by a, b, c, respectively.

(a) A red a+/a+ b+/b+ c+/c+ is crossed with a colorless a/a b/b c/c to give a red F1 a+/a b+/b c+/c. The F1 is selfed. What proportion of the F2 progeny is colorless?
(b) It turns out that another step is involved in the pathway – one that is controlled by gene d+, which assorts independently of a+, b+, and c+.

colorless compound 1 –> colorless compound 2 –> red compound
(d+) (a+, b+, c+)

The inability to convert colorless 1 to colorless 2 is designated d. A red a+/a+ b+/b+ c+/c+ d+/d+ is crossed with a colorless a/a b/b c/c d/d. The F1 corn are all red. The red F1 corn are now selfed. What proportion of the F2 corn is colorless.