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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)?

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I am not a geneticist, but I have some experience as an older (or perhaps just old) scientist in interviewing or examining younger scientists. So I suspect that the interviewer was trying to discover whether the original poster, whom we assume is familiar with modern techniques, was also familiar with the classic methodology of (drosophila) genetics. I therefore suggest that what the interviewers may have been after was simply Genetic Linkage.

Presumably in an interview one would say that one would test for linkage of the trait to a wide range of other traits (eye colour etc.) using the appropriate double mutants. Once having identified a gene showing linkage one would use the genetic map of drosophila (determined by classical methods) to test neighbouring genes, until one found almost ‘complete’ linkage to genes on either side of a candidate.

This answer may miss something that is obvious to a drosophila geneticist. I find it difficult to see how if the gene had already been mapped its phenotype would not also have been described. Perhaps it would have an additional phenotype that doid not involve social behaviour.

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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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  • $\begingroup$ The question states: "no using modern tech". $\endgroup$
    – David
    Commented 11 hours ago

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