Eltab
Lord of the Hidden Layer
in reverse order:If elves and humans can have fertile offspring, and so can orcs and humans, then all three must be subspecies of the same species. … humans should have some quality that makes them compatible breeders with any other humanoid race
half-dwarves
2) Mul (see the Dark Sun Campaign Setting)
1) Usually un-mentioned Human Racial Trait: Prolific Breeder. You can try to have children with a member of any other race you want. Or vice versa.
There was a thread not long ago that got tangled up in the grey area between fantasy and genetics. I think any attempt at 'scientific' or 'realistic' rules simulation is doomed to futility. (Maybe I give up too easily?)
There are arctic terns ringing the far North that can interbreed with colonies near their own but cannot interbreed with colonies from half-way around the Arctic Circle. I don't know how you lay the boundaries between 'species' and various 'sub-species' of these birds. But they might make a good prototype for in-game discussions of mixed-blood children, if the campaign wants to feature such as more than occasional supporting characters.