Most Planetouched are derived from Humans, Tieflings, Aasimar, Genasi and the rest are not exclusive to Human bloodlines. It's the planar part that's dominant, among the lineages. A child of a Tiefling and a Human is a Tiefling, a child of a Genasi and an Orc is a Genasi, a child of an Aasimar and a Goblin is an Aasimar. The influence of the immortal on the mortal bloodline is a strong one that lasts many generations.
As for what happens when different lineages of Tiefling mix, or a Tiefling and an Aasimar have a child, I think that's definitely where one should use the "pick one base species" and apply other aesthetics and possibly background feat choices to reflect that.