Everything Kev said, plus the only caviat is that boys tend to be "sturdier" during incubation, so if any harsh, or out of norm conditions exist during incubation it is more likely to kill the female embryo then the male, but not change the gender of the embryo, that is already set.
That reminds me. there is a mutant gene with the fantastic name of ladykiller. It kills the female embryos.