In a comment thread elsewhere on RPG.SE, someone asserted that tiefling features manifest increasingly down the generations, citing the following text as his evidence:
Tiefling says
Most tieflings never know their fiendish sire, as the coupling that produced their curse occurred generations earlier. The taint is long-lasting and persistent, often manifesting at birth or sometimes later in life, as a powerful, though often unwanted, boon.
Does this text in fact mean that the probability that any given descendant will manifest the taint grows over time?
Does it mean that the chance of the very first generation manifesting the taint is small?
Best Answer
The quoted sentence is not enough to imply that the the chance of manifesting the taint starts low, or that the chance of an individual getting it increases with latter generations. In fact, the chance of it manifesting can go down significantly over time and still make that sentence true. Let's look at an example.
To simplify the math, let's imagine that:
This means that our hypothetical fiend has 2 children, 4 grand, 8 great-grand, 16 great-great-grand, and 32 great-great-great-grand children. On average, how many first generation tiefling children will come from this union?
2 * 0.7 = 1.4
And on average, how many latter generation tiefling children will come of this union?
(2+4+8+16+32) * 0.1 = 6.2
Note that I'm not suggesting that these are the probabilities, only that the quoted sentence holds true even under these probabilities. There may be evidence elsewhere that first generation tieflings are rare...but this sentence isn't sufficient evidence to claim it.