DNA can be a funny thing. Flip a gene here and the resulting offspring is sterile. Twist this one around and the kid winds up twice as big as either parent. And this one? Lets just say that there’s a reason why some genetic experimentation is very, very illegal.
This really comes from an ancient Chinese novel. Imagine during the PCs' travels, they come to a place where the gender roles are completely reversed. What adventures will the PCs encounter in such a place?