This work describes the development and evaluation of a two-session laboratory class based on Tet-on technology and sex determination in the fruit fly "Drosophila melanogaster." The Tet-on system allows conditional control of gene expression, when doxycycline is applied. A laboratory exercise has been developed to illustrate how the Tet-on technology conditionally over-expresses the key sex determining gene transformer (tra) during development and how to inhibit sex differentiation in males, results in a lack of external genitalia and sex comb size reduction. The laboratory practice is inexpensive and straightforward, while allowing students to understand well how molecular biology technology can change biological processes, including development.