Except that genes can be very, very wonky and sex is quite often not a yes/no situation. You'll be fishslapped in the face with this when you finally get going on the biology degree. There was a study done that showed that many if not most people who has their sex reassigned had genetic or hormonal variances to start with. Certainly most of the people I've known who had sex reassignment weren't 100% their birth sex to begin with.