This problem examines possible biochemical explanations for variations of Mendel’s 9:3:3:1 ratio. Except where indicated, compounds 1, 2, 3, and 4 have different colors, as do mixtures of these compounds. A
and B are enzymes that catalyze the indicated steps of
the pathway. Alleles A and B specify functional enzymes A and B, respectively; these are completely
dominant to alleles a and b, which do not specify any
of the corresponding enzyme. If functional enzyme is
present, assume that the compound to the left of the
arrow is converted completely to the compound to the
right of the arrow. For each pathway, what phenotypic
ratios would you expect among the progeny of a dihybrid cross of the form Aa Bb × Aa Bb?
a. Independent pathways
Compound 4
Compound 2
Compound 3
Compound 1
Enz A
Enz B
b. Redundant pathways
Compound 1 Compound 2
Enz B
Enz A
c. Sequential pathway
Compound 1 Compound 2 Compound 3
Enz A Enz B
d. Enzymes A and B both needed to catalyze the reaction indicated.
Compound 1 Compound 2