What would the product be of a human-planetouched mating? I'm playing a Cansin a planetouched from Limbo. If he never finds someone of his own kind, which he is trying to do, he may seek a human mate. I wonder what the offsringing would be. A human or cansin?
Generally, I think it would be a planetouched. At least in the original conception of the tieflings (then the only planetouched race), they were not necessarily a "quarterfiend" - they could be humans with a touch of fiendish blood, or possibly just the descentant of people who had been exposed to the lower planes a bit too much.
I believe the current description of planetouched is people with a touch of planar blood in them. So the two options would be another planetouched offspring, or a straight human with the chance that descendants further down will show up as planetouched.
I'd probably have it as the offspring having a chance of being planetouched; from all descriptions of it, being planetouched isn't a matter of having a certain percentage of outsider ancestory, it's just a matter of having 'some'.
So essentially the question boils down to the old elven subrace crossbreed one, with the same answer: just pick which parrent the child displays more traits of and go with that. Favors the human (or whatever) parrent? Kid's a human. Favors the planetouched? Kid's a planetouched.