homeotic genes A class of genes, including the Hox genes, that play a central role in controlling the early development and differentiation of embryonic tissues in eukaryotic organisms. They code for transcription factors – proteins that bind to DNA and regulate the expression of a wide range of other genes. This binding capability resides in a structural domain of the protein called a homeodomain, encoded by a nucleotide sequence that is characteristic of homeotic genes (see homeobox). These genes were first identified in Drosophila fruit flies, through the occurrence of mutations that alter the development of entire body segments. Drosophila flies have two major clusters of homeotic genes: the antennapedia complex, which controls development of the head and anterior thoracic segments, and the bithorax complex, which governs the development of posterior segments. For example, one mutation of the bithorax cluster causes the thoracic segment that normally bears the halteres (balancing organs; see Diptera) to be transformed into a segment bearing a pair of wings. In vertebrates there are four clusters of homeotic genes located on separate chromosomes.
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