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. At first sight it is difficult to see how if the gene had already been mapped its phenotype would not also have been described. I assume that it would have an additional phenotype that did not involve social behaviour — presumably just the smaller size. In this case any such genes would be candidates for testing for the more obscure phenotype of social behaviour at a specific temperature.
