Mutations in homeotic genes have given scientists insight into how these genes work. One such mutation is the bithorax mutation in Drosophila. The bithorax complex of genes controls the development of the fly's abdomen and posterior thorax. Wild-type flies have three thoracic segments--T1, T2, and T3. T2 produces the fly's wings and legs, whereas T3 gives rise to a pair of legs and halteres (tiny wings). In the bithorax mutation pictured to the right, T3 has developed exactly like T2, resulting in a complete extra pair of wings.

Photo by Ed Lewis