In his book Endless Forms Most Beautiful, Sean Carroll explains the role of tool kit genes in the development of organisms. Tool kit genes express products that in turn regulate whether or not other genes are turned on during embryological development. As such, most of them are transcription factors that bind to regulatory regions of a gene, regions Carroll refers to as switches. What thus determines whether or not a particular gene is expressed during development is the combination of activated and repressed switches as a consequence of the composition of the tool kit gene products.
The teleological echo of all this can be seen from more than one angle.