The other face of pattern

Genetics is founded on the concept that the structures of an organism are controlled – more or less indirectly – by its genes, and since embryos are no exception, there must be genes even for their developing structures. For more than half a century, however, classical genetics was unable to prove their existence, because genes are recognized by mutations, and the mutations of developmental genes normally brings development to a halt, thus making it impossible to observe the effect that they have in vivo. This obstacle was overcome only thanks to some peculiar characteristics of Drosophila melanogaster (a fruit-fly), and to the patient work of Edward Lewis at the California Institute of Technology.
The Drosophila characteristics which allowed the discovery of the first developmental genes are the so-called homeotic mutations that William Bateson had described since 1895. These mutations do not arrest development but transform one part of the body into another.
Antennapedia mutations, for example, transform antennae in legs, which gives origin to an insect with two legs sprouting from its head, whereas bithorax mutations transform the third thorax into a second one, and the insect has an extra pair of wings.
In order to identify the homeotic genes, it was necessary to distinguish them from the other developmental genes, but the mutations of these were usually lethal and prevented the animals from going beyond the larval stage. Lewis noted however that Drosophila larvae have 12 segments which can be individually recognized under the microscope because of a belt of hairs and denticles that has a distinct morphology in every segment. Even if the insects were dying in the larval stage, therefore, the effects of developmental mutations could still be seen in the altered microscopic morphology of their segments. After thousands of crosses and of post mortem examinations of larval cadavers, Lewis was able to trace the loci of bithorax mutations on the right arm of chromosome 3. And soon afterwards the loci of Antennapedia mutations were also found on the same chomosome.