Keep in mind, there is "some" room for play here.
There are things such as Assimars, Tieflings, and Feytouched, all of which are less than half of their "offset ancestry" (anywhere from a quarter to an eighth, or sixteenth), which are still considered, to some degree, divine or mysterious. Depending on how you want to spin elves, that "some degree" could carry it's weight down the bloodline (was the elven line particularly sharp in their eyesight, did they have the gift of elven magic, perhaps the mind of those from this family line has always been particularly strong and able to naturally resist certain things). While largely the individual in question is more human than anything else, the fact they have a unique background lineage in a world where honest pureblood is not typical, then there may be something to that lineage.
Or perhaps nothing at all...
Call a spade a spade, slap a level adjustment on your character, and give them a few natural perks, bonuses and natural stat adjustments. Or don't and just have their background lineage just be a story based thing that comes up at some point, if nothing else, than for story's sake.
It's all either semantics or mechanics anyway.. Heck, in some game worlds, the fact sorcerers exist at all (anyone remember red dragon disciples?) suggests that "somewhere" in ancient history, dragons in mortal forms and humans had casual bedroom encounters. All depends on the game world, the standards, the setting, and the mechanics.