The mean gene is sort of a joke that I creates to describe that certain X factor that tends to make most Dommes, well, Dommes.
I think within many people there's an instinct to poke, to prod, to push boundaries. An instinct that naughty is fun. An instinct to tease. An enjoyment of power and weilding said power. An instinct to take what you want.
As with most instincts it gets sharper if it is honed and dull if it's suppressed. You can call it what you will, a wicked streak, a wild side, etc.
Some people choose to feed it and it grows strong within them. Others choose to bury it since it makes them question if they are "good" while it flows within them.
I find Dommes that are naturally dominant tend to have a hungry and prominent "mean gene." The desire to cause that little bit of extra suffering that they connect with sexually. That's not to say that all Dommes are sadists, but I think most find the notion of a naked and squirming man chained to the bed an attractive prospect. The idea of "I can do what I want and he can't stop me."
I think in most cases of vanilla to D/s conversions that this is where it breaks down. Some of the characteristics are present but the innate desire... that driving hunger just isn't there. In some cases it is dormant and awakens with a vengeance. In others it may never have existed or it has been buried so deeply that it doesn't surface.
To those who embrace their mean gene, thank you.