THE DROSOPHILA SEX DETERMINATION SIGNAL - HOW DO FLIES COUNT TO 2

Seventy years after the discovery that sex in Drosophila melanogaster is determined by the balance between X chromosomes and autosomes, we c an finally identify some of the specific genes whose relative dosage i s responsible for the male/female decision in somatic cells and study how they act at the molecular level Discovery of these sex determinati on genes was delayed because their mutant phenotypes were unanticipate d. It now seems appropriate to consider how the concept of the X/A bal ance may have limited thinking about the fruit fly sex determination s ignal.