On tieflings (a variation from the rules)

I'm having difficulty grasping the Tiefling race as presented in the PHB. I don't want to ban them outright though. So here's the story.

The Tiefling bloodline began among 'native' (as opposed to 'colonist') humans back in the time of the plaque, or perhaps earlier. The dark powers employed by the early tieflings helped them survive the harsh conditions, and also helped there nieghobrs survive. Many native humans have the tiefling gene in their blood.

Unlike the tieflings presented in the PHB, tiefling features do not always appear in tiefling children. They are in nearly every way normal humans, and they blend in quite well. On rare occassion, a true tiefling is born among the native humans, with horns and tail. Families in which this occurs are shunned.

So in my conception, tieflings are not so much a "race without a realm", but instead are a mutant strain within humans from the new world.