Clearly genetics cant be a huge part.
If it were so the people living in former long time dictatorships or comming from a slave background would show significant sub tendencies. Since assertive or dominant population would be reduced. Atleast thats my assumption.

I have seen some tendency towards both sub and dom's having been bullied as children but that could be for being different and not an actual cause.

Personally i had rather dominant parents and a somewhat absent father. My sub have a dominant mother but really a "normal" father. I recall fantasies of torturing and humilliating people as far back as i can remember (age 5-6). But back then my ideas of having victims restrained on conveyers and fantasising about their fear wasnt sexual pr say.