@Dexai yes, you are right, it was one of the changes that happened during 4e. Previously it was normal for tieflings to "breed out" with normal mudane races and eventually the family would resemble normal mudane people, and then suddenly sport a tiefling somewhere down the track when the fiend blood would get active somehow, or get lucky on the roulette wheel of genetics. It was a fairly common backstory for making warlocks, fiendish blood in your ancestry. I actually had a tiefling alchemist with a similar story (we were playing 5e, but not playing in the realms, so I thought it would be fine) that unfortunately got given away when his human parents realised their baby had horn buds and a little tail.
But now there's no such thing as diluting the blood, tieflings always breed tieflings, regardless of the race of the other party. (with obvious exceptions for races that are incompatible in that way, or with fiends). Or, as I like to say "It's Tieflings all the way down".
Which you know, as well as being kind of strange, also begs the question of if tieflings always breed true, does that meant he population of tieflings is exponentially increasing?
