In my D&D 5e campaign one of the PCs who's a wood elf started dating a dark elf, and as a result she got pregnant and she is about to give birth. One of the other players had their PC die and he is considering playing as the child of the couple.
My question is, which race is the child going to be?
I don't believe 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, when subraces interbreed, the offspring just have the traits of one of their parents. 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 choose the subrace of one of the parents. and take their racial traits. If this character might be a PC it seems fair to let them choose which they'd like the character to be; otherwise, I'd just flip a coin.
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.
This is a late answer, but in terms of inheritance: while with most different subraces, it comes to a flip of the coin essentially on which of the races the child takes after - this is explicitly not the case when it comes to both original Dark Elves and Drow.
Rather:
When different sub-races of elves intermarried, there was an equal chance for the child to inherit either of the parents' traits. However, children born of the union between a drow and elf were far more likely to inherit the drow traits, and were more likely to parent a drow themselves. Dark elves possessed this traits too.
My citation here being the Forgotten Realms drow page,
Drow and in this case Dark Elf heritage, will be dominant over the wood elf in terms of abilities and appearance. Although that's not to say, there won't be any slight differences that would tip off to a different heritage, such as perhaps eye colour (the same wiki page, listing some differing eye colours as an indicator of some surface elf heritage somewhere in the bloodline).
But it's in fact so dominant, that Vhaeraun encourages his followers on the surface to take advantage of it:
He saw a general need for advancement for elves and encouraged cooperation, including intermarriage among the elven races. The goal included the subjugation of other races. Intermarrying also had an ulterior motive in the form of increasing drow numbers on the surface by taking advantage of the drow's genetic dominance
Source this time being his forgotten realm page, which itself provides several citations.
And as shown in the initial quotes: Dark elves share this "very genetically dominant trait".
So by far, the most likely outcome is essentially a dark elf baby with perhaps minor cosmetic such as eye colour differences.
The only real answer is "whatever you decide". I can't imagine any kind of official ruling on such a question, other than maybe "flip a coin".
However, here's a potentially useful option: don't decide. Let the player do that through RP...find some roleplaying options that would give the character a chance to express one side or the other more. This isn't just in terms of good vs. evil, but also environment. Does the young elf seem more comfortable in the forest, or underground? Do they feel some kind of kinship to spiders, or a loathing for them? Let the player come up with answers to those sorts of questions in play, and have those answers determine which set of genes will ultimately express itself more strongly as the child moves through puberty and into adulthood. I think that might be a rather more interesting...and in a world full of magic, thematic...way of finding your answer.
What are racial subtypes in your world?
In mine, racial subtypes are culture; that is, the shared features are part of the creature's innate physiology, whereas the non-shared features are imparted by cultural experience.
For example, in my world, all dwarves have innate instincts about stone (Stone-cutting), and their physiology makes them hardier and poison resistant(Constitution bonus, Dwarves Resilience), as well as slower and able to see in the dark. Mountain Dwarves live under a stricter (and more racially homogenous) society, which includes mandatory military service(Strength bonus and armor proficiency); whereas Hill Dwarves are healthier due to living in fresh air, and their expanded experiences(with others, in less controlled and organized society, looking up) makes them wiser and more perceptive(Wisdom bonus).
As a more direct example, a High Elf's Fey Heritiage, Keen Senses and Trancing would be innate, but it would learn how to use "Elf Weapons", use a single cantrip, and and this would.
In your case, I would say that the character is an Elf, and based on what happens to it, it can gain any of the Elven subtypes. (Yes it is possible for a Wood Elf and a Dark Elf to have a kid who goes away to college and comes back more Intelligent and High :P)
