I'd recently attended an interview and was asked a question I've been trying to get to the bottom of. The question was that there is a mutation in which some smaller drosophila are only attracted to other smaller ones. This is activated at a specific temperature. Through drosophila crosses only, given that you have access to balancers and whatever else you need, how would you pinpoint which gene exactly is causing this mutation?

A test cross would tell you if it is genetic, and if it may be dominant or recessive, but how does one pinpoint a specific gene only through crosses (no using modern tech)?

  • $\begingroup$ Shouldn’t your title be “phenotype” rather than “genotype” (or perhaps the more flexible, “behaviour pattern”)? $\endgroup$
    – David
    Commented May 21 at 20:12
  • $\begingroup$ ah yes, you're right my bad - I meant to say phenotype, that was a typo $\endgroup$ Commented May 24 at 4:35

1 Answer 1


Not sure if this is what they'd be looking for, but if you were allowed to do research ahead of time you could select a few genes that you think might be involved in this behaviour, then use RNAi and GAL4/GAL80 to temporally knock down your genes of interest at high temperatures to see which genes give you the phenotype when knocked down


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