Gap gene

A gap gene controls the shape of a developing zygote early on, producing (as the name implies) a gap in an otherwise heterogeneous form. One example of this is the Kruppel gene, which is a sort of gatekeeper for a number of other genes, regulating their activity. Gap genes encode for transcription factors.

In Drosophila, such genes include hunchback, giant, kruppel, knirps, and tailless.