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Homeotic genes set up the basic regional layout of an organism, so that eyes form on the head and not on the abdomen, and limbs form at the sides and not on the head. Even a single mutation in the DNA of these genes can have drastic effects for the organism (see Homeotic Mutants, below), and so these genes have changed relatively little over time.
Antennapedia is a hox gene first discovered in Drosophila which controls the placement of legs. A loss-of-function mutation in the regulatory region of this gene can result in the development of the second leg pair into ectopic antennae [pair of legs growing from head].