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Antennapedia is a HOM-C gene first discovered in Drosophila which controls the formation of legs during development. Loss-of-function mutations in the regulatory region of this gene result in the development of the second leg pair into ectopic antennae. By contrast gain-of-function alleles convert antennae into ectopic legs.
This is just one illustration of the tendency of organisms to exhibit variations on a theme, modulated repetition. Legs and antennae are related to one another as much as molars are to incisors, fingers are to toes, and arms are to legs.
Antennapedia also refers to a gene complex in Drosophila which is responsible for formation and differentiation of the thoracic and head segments of the fly's body.