"Drosophila melanogaster" is a commonly utilized organism for testing hypotheses about inheritance of traits. Students in both high school and university labs study the genetics of inheritance by analyzing offspring of appropriate "Drosophila" crosses to determine inheritance patterns, including gene linkage. However, most genetics investigations with "Drosophila" analyze offspring patterns of the F2 generation of dihybrid crosses to determine that genes are linked but do not calculate the map units between the linked genes. Calculating map units between linked genes is most straightforward when testcross data is used. However, setting up a testcross is not trivial. Constructing a testcross in "Drosophila" requires obtaining an F1 virgin female fly to mate with a homozygous recessive male fly in order to produce the subsequent generation for analysis of traits. In a teaching lab setting in which there are severe constraints on lab time, students have great difficulty in obtaining virgin F1 females to set up the testcross to generate data for mapping. This article describes how to use F2 data generated from an F1 sibmate cross to determine map distances in linked genes. (Contains 1 table.)