Alfred Henry Sturtevant is another of my all time heroes of Drosophila genetics (see also Calvin Bridges).
As an undergraduate he generated the first genetic map of any organism. He realised the linear order and relative position of genes on a chromosome would be reflected in the frequency of genetic recombination between them. Using a set of six X-linked mutants (actually two were alleles, so there were only five markers), he assembled a 5 locus map of the X chromosome. [Sturtevant (1913) J. Exp. Biol. 14; 43-59; pdf]
• Sturtevant's discovery of inversion is important because it explains the why/how of certain genetic defects and discovery of inversion allows for its presence to be tested for