Can the different sub-races of a particular race interbreed, like a drow with a wood elf, or a hill dwarf with a mountain dwarf? If so, would the child be a half-subrace from each parent? Is this possible in D&D?
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I don't know if there's more modern material to reference, but this specific question is addressed in 3rd edition Forgotten Realms setting books (and FR material is heavily referenced in the core 5e handbooks). According to Races of Faerun's introduction, under the Race, Subrace, and Ethnic Group heading:
In game terms, a subrace is, for all intents and purposes, a race of its own, but all the subraces of a particular race share many of the same qualities and are related. When parents of different subraces have a child, the child almost always "takes after" one parent or the other.
Basically, yes, subraces can interbreed, but the offspring have the traits of one of their parents rather than being a hybrid. The distinction between subraces is deeper than the distinction between ethnic groups, who have varying appearances but identical mechanical racial traits; subraces "breed true" rather than mixing in most circumstances. I'd be inclined to say a mixed subrace individual would appear to be one particular subrace, but could conceivably have a feature or two that hints at their more diverse heritage, say eye or hair colour. Mechnically, however, you just choose the subrace of one of the parents.
Note that my source is from 2 editions ago and is technically a setting-specific source, but there's nothing equivalent available for 5e yet and as far as I can tell the specific question of what happens when subraces interbreed is not raised in other material. There's no particular reason to think this should be different in the newer edition or different settings, especially as it's simply the easiest way to adjudicate the scenario. Of course, your GM is free to decide that things work differently in their world.
The various races can usually interbreed, so the subraces should be far more fertile.
Traditionally, such a half-n-half is treated as one or the other of the parents' subraces.