Discussion
Two genes X and Y are involved in root development in such a way that all the following three homozygous mutants show rootless phenotype.
(i) Gain-of-function mutant of X
(ii) Loss-of-function mutant of Y
(iii) Double mutant of gain-of-function of X and loss-of-function of Y
Which one of the following genetic pathways best describes these observations?
Answer: A
The pathway is X suppresses Y and Y promotes root development.
Test it on each mutant. A gain-of-function X over-suppresses Y, so Y activity is low and roots fail to form: rootless. A loss-of-function Y removes the promoter of root development directly: rootless. In the double mutant Y is already absent, so the extra X makes no difference: rootless. All three observations fit.
If X promoted Y and Y promoted roots, gain-of-function X would give more roots, not none. If Y suppressed roots, losing Y would give roots, which rules out both pathways in which Y is a suppressor. The double mutant also tells us X acts through Y, because it looks like the Y mutant.
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