sptlt201 webOne cell. One organism. One fate: male, or female. The way Nature designs things, you would expect traits as fundamental as those that make a boy a boy or a girl a girl, for instance, to be inscribed in their respective DNA from the very start. In a way they are, of course, but there is a subtlety: if what defines gender is for some reason missing, then the other sex may emerge almost by default. So the process is far more than a simple case of gender that is genetically predetermined. The same goes for Musca domestica, or the common housefly, as it does for many other animals. There seems to be a sort of switch which, when on or off, will produce one sex or the other. This particular part of the sex decision system is preserved in many organisms, and scientists are beginning to realise that the upstream mechanisms leading to the definitive sex switch are surprisingly diverse and, in the case of Musca domestica, can even change from fly to fly. But one thing is preserved: to become male or female, you need a factor that will ultimately push the on/off sex switch. In the housefly, that factor is Mdmd, or Musca domestica male determiner. Read more