A group of three adjacent homeotic bithorax genes forms the bithorax complex (BX-C) genes.
This was first described in Drosophila by Calvin Bridges in 1915. A mutant bithorax gene results in two pairs of wings in the thoracic segments of the fly.
Word origin: Latin bi- (a combining form meaning “twice”, “two”) + Latin thōrāx (breastplate, chest).
See also: homeosis.