How does threshold effect of temperature sensitivity (like for R7) work?
A temperature sensitive allele shows the mutant phenotype at the restrictive temperature (24.3oC in this case). At the permissive temperature (22.7oC), the mutant protein is wildtype and functional, but just barely. It’s ready to unravel like a genetics student in the last ten minutes of the first exam. So with such a shaky protein (“sensitized background”), there is decreased pathway activity, but it is still above a critical threshold/barrier between a wt and mutant phenotype. Any additional mutation which decreases the pathway levels downstream will cause a mutant phenotype at the permissive temperature, because the pathway activity has dipped below the wt/mutant threshold.
