In the mouse/fly experiment that Madison referred to, the mouse Pax6 gene is used in place of the fly eyeless gene (a copy of the homolog to Pax6 ; fruit flies have two homologs). The gene codes for a transcription factor, a protein that controls expression of other genes. Explain how a mammalian gene can successfully induce development of a compound eye in fruit flies.

Pax6 is an evolutionarily conserved transcription factor that activates genes involved in eye development in several animal groups ranging from vertebrates to invertebrates

Explanation:

The Paired box 6 (Pax6) gene is a conserved transcriptional factor involved in regulating eye development across bilaterian species. This homeotic master gene may also play different functional roles in different animal groups. For example, in vertebrates, Pax6 is also required for the formation of the central nervous system (CNS) and pancreas. In invertebrates, Pax6 homologs are responsible for the development of nonocular head structures. In Drosophila, there are two homologs of the Pax6 gene, eyeless (ey) and twin of eyeless (toy), which are involved in eye specification and cell proliferation, respectively.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.