Yes, there is such a thing. I dated a girl a couple of years ago, and we switched up the top, bottom roles a few times. Also, it's easier for a sub to switch or learn to be a dom because they know first hand what being in that position feels like. It's harder for the dom, as their ego or mind set is more difficult to shift into a sub role. It's just a matter of how easy it is for a dom to shift roles, it does happen though. On a more basic standpoint, men and women both have masculine / feminine traits and depending on certain situations, one or the other will be displayed more prominently.