The adenine biosynthesis pathway in the budding yeast S. cerevisiae has some unique features. If the ADE2 gene is non-functional, the yeast cells will build up a reddish pigment and appear red, rather than the usual white color. Other members of the biosynthesis pathway, such as ADE5 or ADE17, show no such visual coloration phenotype and are only visually identifiable by their inability to grow on media lacking adenine.
Note: In yeast, wild-type genes are written in all caps, while mutant genes are written in lowercase.
In examining double mutants of these genes, you find the following results:
ade5 — ade17 double mutants are white.
ade5 — ade2 double mutants are white.
ade17 — ade2 double mutants are red.
Based on this information you can determine the following facts about the pathway.