whole debates can be started on this whole subject, I categorize each Tiefling parentage into 4 different subgroups
REAL TIEFLINGS/BIO-TIEFLINGS:
Succubi/Incubi + Human = Cambion/Alu-Fiend
Cambion/Alu-Fiend + Human = Tiefling
FAKE TIEFLINGS/PACT TIEFLINGS:
Human + Human + Pact Curse = Tiefling
TIEFLING TIEFLINGS/BOTH TIEFLING PARENTS:
Tiefling + Tiefling = Tiefling
HALF TIEFLINGS:
Tiefling + Human = Tiefling
all are Canon in 5E BTW.
The question is: Does the offspring of a Cambion or Alu-fiend and a human always result in a tiefling being born or can the tiefling actually be one or more generations later? The PHB isn't exactly clear on that point as it just mentions you have an infernal ancestor somewhere in your bloodline.
