Bithorax complex

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.