In a paragraph in my dissertation, I explain a (supposedly) pre-theoretical approach to mixed race that made sense to me when I was a kid. It seemed to me that a helpful way to explain what I would have thought (and what many Americans seem to me to think) is sort of parallel to the way Tolkien speaks of half-elves in his fictional world. In the process, I realized how Tolkien speaks of this is much more complicated than I'd though, and I couldn't in good conscience leave it the way I had initially stated it, so it led to a long clarificatory footnote that I thought a number of the readers of this blog might appreciate for its geekiness.
Here is the sentence in the text of my dissertation that led to this:
I confess that this is how I thought of these matters in my unreflective, supposedly-pretheoretical analysis of things in high school. I would have taken a Barack Obama to be half-black in the same way that I took Elrond in Tolkien's Lord of the Rings to be half-elf, his daughter Arwen to be three-quarters elf, and her children with Aragorn to be three-eighths elf.
I realized there was a complicating factor, though. Aragorn had an elvish ancestor as well, and I wanted to check to see how far back it was. In the process, I was reminded that Elrond himself wasn't the product of a full elf and a full human, and it led to a much longer and much geekier footnote than I ever expected to be putting into my dissertation, but it's hard to be fair to Tolkien without acknowledging this, and it turns out to illustrate a different phenomenon in how racial classification works in some places outside the U.S. Here is the footnote as it stands now:
Tolkien buffs may quibble here, and they would be right to, for two reasons. (1) Aragorn was the sixteenth in the line of Elros, Elrond's brother, and thus he himself has elvish ancestry, even if minuscule (I believe one over two the thirty-second power). (2) Elrond and Elros themselves weren't exactly half-elves to begin with. Their father was actually half-elf, and their mother was one-fourth human, one-eighth Maia (a kind of lesser angelic-like divinity), and five-eighths elf. That would make Elrond and Elros nine-sixteenths elf, three-eighths human, and one-sixteenth Maia. Arwen's son twenty-five sixty-fourths elf, by these measurements, not the three-eighths that would result if Elrond were literally half elf. But we get the language of half-elves for a number of Tolkien characters with mixed ancestry, regardless of actual percentages. What this suggests is that the culture of Tolkien's world seems to treat someone as half-elf for having any level of mixed ancestry, eschewing a one-drop rule in either direction but insisting on little expression of nuance or gradation among those labeled half-elves. This would presumably operate something like the label 'brown' in some Latin American and Caribbean countries, applying to anyone of mixed heritage regardless of the particular number of ancestors of each race.
This doesn't (at this point) make it into my dissertation, but compare Rowling's terminology in the Harry Potter books. There are two systems of classification, the one that is dominant until Voldemort's rise to power (and presumably again afterward) and the one operating during his reign of terror. In the method of classification that we learn throughout most of the series, someone with a magical parent and a Muggle parent is a half-blood. Harry's mother, Severus Snape, and Voldemort himself are half-bloods. Someone with no magical parentage but who has magic is called a Muggle-born. But in the generation after a half-blood, if the other parent is magical, there is no discussion of being a half-blood. Harry himself is never called a half-blood by anyone in the mainstream of wizarding society. There's no one-drop rule in either direction, but there's a sufficient-drop rule apparently, because once you get to three-fourths magical parentage you're no longer consider partial, and even if you have no magical parentage you're treated as magical in one sense. It's what you can do and not your parentage that makes the difference in terms of the law.
But then there's Voldemort's regime. Muggle-borns are Mudbloods by the pureblood mindset even before Voldemort's return to power, but once he takes control of things they simply become Muggles. They're assumed to have stolen their wands, because they're not magical. Half-bloods (other than Voldemort and Snape) are sometimes called Mudbloods, and Harry (who had a full magical parent and a Muggle-born magical parent) is considered a half-blood, because his mother was a Muggle.
There's something more like the one-drop rule operating here, although not quite. But neither of these systems of classification works out quite like Tolkien's. And keep in mind that elves in Tolkien don't think of humans as corrupting or impure. A half-elf can choose to be mortal or to be an elf in ways that don't involve just legal status. It affects whether they become mortal. Arwen, with much more elf ancestry than human, still could chose to become mortal. There's nothing parallel to that in Rowling's classifications. Perhaps if we had enough evidence for how half-orcs were classified (there are only a couple suggestions in Tolkien that there are such things but no clear cases where it's more than just simple one-human, one-orc parentage). If he treated three-fourths orcs as half-orcs in a case where the human is mixed with something seen as corrupting, we'd have a good test case for whether his principle would expand to other cases of mixing. But I know of nothing in his fictional world that gets any more complex than simple one-one mixing except when it comes to elves (and the one Maia in the line of Elrond).