Start thinking like that, and you wind up with the idea that cross breeding two Half-Elves should result in one Human, one Elf and two Half-Elves because of the punnit square.
That would work only if only a single characteristic was involved and none of the alleles were dominant... Which most probably isn't the case, although most authors handle it as if it were, or so it seems with all those half-elves out there that are carbon-copies of each other. (On the other hand, in these stories not only the half-elves are clones...)/mini-rant
Anyway, I think you're right. Apart from the fact that it's damn hard to make fantasy races work genetically according to Mendel's laws, it's much more fun to devise completely new genetics for them. And who said they have to behave genetically like us?
In one of my stories, I actually worked out a set of fifteen or so genes, each with two or three separate allelles, that caused the mix of magic talents in my story. I've lost the info on the specific genes, but I do recall that it was possible for a child with a strong talent in this one plot-important 'forgotten magic' to pop up without any relatives having that particular talent. I was planning to have a generations-long conspiracy in place based on the faint hope of finding the hypothesized decendant of this one person, only to discover that someone who is most definitely *not* descended from them shows the qualifications they need.