This question already has an answer here:
How should I justify different intelligent species being able to interbreed (and potentially produce healthy and fertile offspring) in a fantasy setting, especially if I want it to not just boil down to "The author said so"?
Considering some real-life examples, like humans with dwarves and elves it seems feasible, as one could just say that they're closely related, like how Neanderthals interbred with modern man, closely enough that it's not an issue, but what about when that's not the case?
Or when it's between a human and something harder to justify a relatively close relation with, like a troll or ogre/orc or goblin/merfolk or something?
Or what about in situations like human/dragon hybrids (assuming that shapeshifting doesn't change you genetically into the species you're becoming), or human/merfolk? I was actually inspired to ask by thus by reading on TV Tropes about D&D's examples of hybrids.