Essential Genetics 6th Edition by Daniel Hartl

Exercise 6

This problem illustrates how conditional mutations can be used to determine the order of genetically controlled steps in a developmental pathway. A certain organ undergoes development in the sequence of stages A ?B ?C, and both gene X and gene Y are necessary for the sequence to proceed. A conditional mutation X ' is sensitive to heat (the gene product is inactivated at high temperatures), and a conditional mutation Y ' is sensitive to cold (the gene product is inactivated at cold temperatures). The double mutant X '/ X '; Y '/ Y ' is created and reared at either high or low temperatures. To what stage would development proceed in each of the following cases at the high temperature and at the low temperature?
(a) Both X and Y are necessary for the A ??B step.
(b) Both X and Y are necessary for the B ?C step.
(c) X is necessary for the A ?B step, and Y is necessary for the B ?C step.
(d) Y is necessary for the A ?B step, and X is necessary for the B ?C step.