Figure 14.22: Inhibition of a Molecular Switch Results in No Eyes (A) In the Mexican tetra fish (Astyanax mexicanus), fish dwelling on the surface have two eyes (left), whereas those living in dark caves have no eyes (right). The difference results from overexpression of the Shh gene in cave-dwelling fish (B), which inhibits production of the molecular switch made by the Pax6 gene (C).