Chapter 19, Problem 9P

Sevenless is an unusual receptor protein in that it is required only in one nonessential cell type—R7 precursor cells. That is, sev− flies (null mutants) lack R7 cells in their eyes, yet they are fully viable and fertile under laboratory conditions. In contrast, the epidermal growth factor receptor (EGFR) is required in a variety of cell types beginning early in development. Because of the pleiotropic function of EGFR, flies that lack this protein die during embryogenesis.

a.	The Ras/MAPK pathway that relays the Sevenless signal to the nucleus (Fig. 19.6) also operates downstream of EGFR. Explain why the screen for modifiers of sev mutations used to identify components of the Ras/MAPK pathway (Fig. 19.5) would not have worked with Egfr mutations
b.	Design a modifier screen that would identify mutations in genes in the Egfr pathway. (Hint: Use a transgene.)