I can't believe that people are arguing about genetics as if that's even a thing in a fantasy setting.
D&D species can interbreed or not interbreed, have DNA or not have DNA, be made of flesh or not made of flesh, etc, etc, as the DM wants, as the World Lore goes, and no one can tell anyone else otherwise.
It's all made up.
And I'm getting tired of the "slippery slope"-type arguments I'm seeing around here. Just because someone might want to mix an elf and a gnome doesn't mean that they'd want to mix an Aasimar and a Tortle. (Or perhaps they would: Koopa Paratroopa, anyone?) And if they DID, that would be for their own game, and under the rules set by their DM, their table preferences, or their game world, depending on what they care about. And no one can tell them otherwise.