Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

Exercise 5

The Eyes Have It. For this question, please consult Box 14B. Recall that each ommatidium in the compound eye contains a photoreceptor cell, R7, which must receive a signal from a neighboring cell, R8, to differentiate. This pathway depends on the Ras signaling pathway. Also, recall that mutants provide insight into this pathway. For each of the following situations, indicate whether the R7 cell would be expected to differentiate. In each case, clearly state your reasoning, based on your understanding of the molecular pathway involved.
(a)R7 cells are mutated so that their Sevenless receptors lack the SH2-binding domain.
(b)R7 cells are mutated so that they lack functional Sos but contain a constitutively active, dominant mutation in Ras.
(c)R7 cells are mutated so that they have no functional MAP kinase.
(d)R8 cells are mutated so that they produce too much of the Bride of sevenless ligand, and R7 cells lack Sevenless.