Complex sentence: The fruitless gene is a "Drosophila" gene that encodes transcription factor proteins required for proper development of parts of the fly nervous system and body required for fly sexual behavior.
Split into simpler sentences: The fruitless gene is a "Drosophila" gene that encodes several transcription factor proteins. Fruitless transcription factors are required for proper development of several body structures, including neurons and cells necessary for fly sexual behavior.