All animals with eyes or eyespots that have been studied so far share a gene in common. When mutated, the gene Pax-6 causes lack of eyes in fruit flies, tiny eyes in mice, and missing irises (and other eye parts) in humans. The sequence of Pax-6 in humans and mice is identical. There are so few sequence differences with fruit fly Pax-6 that the human/mouse version can cause eye formation in eyeless fruit flies, even though vertebrates and invertebrates last shared a common ancestor more than five hundred million years ago. Fruit-fly eyes are of the compound type, which is structurally very different from the camera-type eyes of mammals. Even the camera-type eyes of molluscs, such as octopi, are structurally quite different from those of mammals. Yet, fruit flies, octopi, and mammals possess very similar versions of Pax-6. The fact that the same gene helps produce very different types of eyes is most likely due to (1) the few differences in nucleotide sequence among the Pax-6 genes of these organisms (2) differences in the control of Pax-6 expression among these organisms (3) the independent evolution of this gene at many different times during animal evolution (4) variations in the number of Pax-6 genes among these organisms

Pax-6 is a member of the Pax gene family which is responsible for carrying genetic information that will encode the Pax-6 protein. it acts as a master control gene for the development of eyes and other sensory organs, certain neural and epidermal tissues as well as other homologous structures usually derived from ectodermal tissues. however it has been recognized that a suit of genes is responsible for eye development and therefore the terms of master control gene may be inaccurate.