Genetic Control of Development

Homeotic genes also control development in other species, from yeast to humans, although the details are not as clear as they are for the fruit fly. Homeotic genes, called Hox genes, control the development of segments in the mammalian hindbrain, for example, and help establish the anterior-posterior and dorsal-ventral axes in the limbs. Vertebrates have duplicate copies of the Hox genes on several chromosomes, all of which function together to specify, for example, limb development. The multiple copies provide a redundancy not found in the fruit fly, thus making the effect of individual genes harder to detect. Nonetheless, by "knocking out" multiple versions of a particular Hox gene, researchers have shown their dramatic effects. For example, mice that are missing two copies of Hox 11 have no forelimbs, and the wrist is fused directly to the elbow.