@a_breakin_glass @Elizafox

I mean, there are genes *not* on the X or Y chromosomes which code for masculinizing and feminizing hormones, and sometimes you get recombination which results in masculinizing hormones on the X chromosome or feminizing hormones on the Y. More than that, mosaicism & chimerism can happen, resulting in a person who has both XX and XY cells.

And that's just the *genetic* portion of the puzzle. There are environmental factors that can skew gene expression toward masculinizing XX or feminizing XY fetuses.