A particular problem with winged dragons is that they have too many limbs for a regular reptile — that is, wings in addition to four fully developed legs. All other land vertebrates never have more than four limbs, arranged in two pairs, fore and hind, and the wings of flying vertebrates are invariably modifications of the forelimbs.
The simplest solution is to suggest that an extra pair of wings is the product of some kind of mutation. English geneticist William Bateson (1861-1926) realized that there was a special category of monster in which organs such as legs or wings developed perfectly normally but were abnormally duplicated, or in the wrong place, and he coined the word ‘homeosis’ to refer to this phenomenon.
More than 80 years passed before the genetic basis of homeosis was understood. Working at the California Institute of Technology in Pasadena, Edward Lewis found that homeotic transformations were linked to a distinctive cluster of genes — at least in the fruit fly Drosophilia melanogaster
, the experimental animal of choice in genetic laboratories for more than a century.
Disruptions to one or other of this gene cluster leads to distinctive mutations in which legs develop in place of antennae, for instance, as pictured here.