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I don't get this problem, at all.

Assume that two pigments, red and blue, mix to give the normal purple color of petunia petal. Separate biochemical pathways synthesize the two pigments, as shown in the top two rows of the accompanying diagram. "White" refers to compounds that are not pigments. (Total lack of pigment results in a white petal.) Red pigment forms from a yellow intermediate that is normally at a concentration too low to color petals.
Pathway I ... → white1 →E blue
Pathway II ... → white2→A yellow→B red
Pathway III ... →white3 →D white4
( the letters should be on top of the arrows, and the numbers are subscripts)
A third pathway, whose compounds do not contribute pigment to petals, normally does not affect the blue and red pathways, but, if one of its intermediates(white) should build up in concentration, it can be converted into the yellow intermediate of the red pathway. In the diagram, the letters A through E represent enzymes; their corresponding genes, all of which are unlinked, may be symbolized by the same letters. Assume that wild-type alleles are dominant and encode enzyme function and that recessive alleles result in a lack of enzyme function. Deduce which combinations of true-breeding parental genotypes could be crossed to produce F2 progeny in the following ratios:
a. 9 purple: 3 green: 4 blue b. 9 purple: 3 red: 3 blue: 1 white c. 13 purple: 3 blue d. 9 purple: 3 red: 3 green: 1 yellow
(Note: No mutations, blue mixed with yellow makes green)

Please help!

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