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The Bithorax complex (BX-C) is a group of homeotic genes in Drosophila melanogaster which are believed to control the differentiation of the abdominal and posterior thoracic segments, located on chromosome III. When these genes are mutated, the third thoracic segment becomes a repeat of the second thoracic segment, creating what is essentially a second thorax. This can result in a second pair of wings, a second stomach, and duplicated thoracic features in varying degrees.
The complex includes Ultrabithorax (Ubx), Abdominal A (abd-A) and Abdominal B (Abd-B).
Calvin Bridges discovered Bithorax (mutation/gene) in the fly in 1915; after that, Edward B. Lewis named and worked on the Bithorax complex and proposed a model in a classic Nature paper in 1978 and was given a Nobel Prize.