Suggestion that these are master control genes for eye morphogenesis. Four other Drosophila mutants that lead to same phenotype do not affect expression of ey, suggesting that it is a developmental switch. Homeotic mutations identify master control genes that specify body plan along antero-posterior axis of flies. Antennapedia and Bithorax clusters as examples. Gain of function illustrated by replacement of antenna with a middle leg.
Loss of function is lethal with transformation posteriorly.
If it is a master control gene, ectopic expression should induce formation of eye structures in other body parts. Suggests ey is master control gene for eye morphogenesis. All are epistatic to ey - it is the top of the regulatory cascade. Proves role of ey as a homeotic gene - overexpression leads to gain of function result like classic homeotic genes. Conservation of sequence between ey, Sey and Aniridia suggests common function.