Slowly but surely, some of the mysteries about how gender is determined in animals are being solved. It is absolutely not a simple issue of one set of chromosomes or another. For example, take this fascinating piece of research. By altering a single gene, scientists at the Institute of Molecular Biology in Mainz have been able to cause mice that have male chromosomes to develop physically as females. Obviously this currently applies only to mice, and no one is going to do experiments of this sort on human embryos, but the mere fact that it happens ought to be enough to give the lie to the people who yell about how intersex people are “not natural”.