FIGURE 1-30 Homologous genes regulate eye development in diverse animals. (a) Development of the large compound eyes in fruit flies requires a gene called eyeless (named for the mutant phenotype). (b) Flies with inactivated eyeless genes lack eyes. (c) Normal human eyes require the gene Pax6, the homolog of eyeless. (d) People lacking adequate Pax6 function have the genetic disease aniridia, a lack of irises in the eyes. Pax6 and eyeless, which encode highly related master transcription factors that regulate the activities of other genes, are homologs and presumably descended from the same ancestral gene.

[Parts (a) and (b) Courtesy Andreas Hefti, Interdepartmental Electron Microscopy (IEM), Biocenter of the University of Basel. Part (c) © Simon Fraser/Science Source. Part (d) © Mediscan/Alamy.]