#see i'm a nerd about this because i've made so many tief ocs

[powdermelonkeg]

Wouldn’t one of the parents have to be a tiefling in order for the infernal heritage to pass down in such a way? Like it’s not a recessive gene scenario where both parents can be carriers, or a half-elf thing where it sort of splits the difference - if there’s infernal blood strong enough to show, it’s showing on parent AND child entirely. Elven parentage would get overwhelmed by the infernal half, so they’d follow normal tiefling rules

Counterpoint, Player's Handbook, 5e:

To be greeted with stares and whispers, to suffer violence and insult on the street, to see mistrust and fear in every eye: this is the lot of the tiefling. And to twist the knife, tieflings know that this is because a pact struck generations ago infused the essence of Asmodeus—overlord of the Nine Hells—into their bloodline.

Say warlock grandmum swore her blood to the devil she pacted with. Grandchild born after comes out as a tiefling. What then?

Counterpoint 2, Planescape: Torment, computer game from 1999, 2e:

Eh... She's a tiefling, chief. They got fiend blood in their veins, usually 'cause some ancestor of theirs shared knickers with one demon or another. Makes some of 'em addled in the head... and addled-looking, too.

Both counterpoints, whether you believe that a pact could cause it or if it's purely genetic, imply that there's several generations before the devil blood shows up.