The Sil is honestly my weak spot
Tolkien knowledge wise. I've read it, but not enough times to really
know the characters enough to determine a favorite, most hated, etc. That being said, to reply to the above:
I've done a good deal of study on genetics and genealogy. As you said, the
Elves were highly inbred, and it's always struck me how little Tolkien showed the impact of inbreeding within the characters. Elves, typically speaking, are intelligent, wise, long lived, with perfect health - so on and so forth. The list goes on and on, showing a
race that seems on the surface to be the best and the brightest of the
Middle Earth races.
I do note three things, however, that show that Tolkien was not completely blind the the impact of inbreeding.
1) Pale features, blond hair, and blue eyes are all recessive traits, genetically speaking. This means that in order to keep this the common feature between members of a family, both parents and both sets of grandparents would have to be pale, blond, and blue eyed in order for there to be an odds above 50% that the offspring would carry all of those traits. Given that Elves are nearly always described with these features, this is a perfect show of the impact of inbreeding.
2) By the end of
the Third Age, Elves were leaving Middle Earth. During
Lord of the Rings, Arwen becomes ill as the power of
the Ring grows. While the films implied that it was the corrupted power of
the One Ring, wielded by
Sauron, which would cause Arwen's death. I believe that the Ring in Sauron's hands itself would not cause an
Elf to die, but rather that with the power of the Ring, the immortality of the Elves would fail, reverted them near enough to human state. Without said immortality, the poor health of massive inbreeding would become readily apparent. By the time
Lord of the Rings was going on, Arwen had not yet made her choice (to go with her father into the West or remain in Middle Earth as a mortal), so she was being "judged" per her elven status. The only other Elves in Lord of the Rings who seemed to not being high tailing it out of Middle Earth during the war were the bearers of the three Elven
Rings of Power, which I'd imagine would give them a bit of a buffer against the rising power of Sauron.
3) Now let's look at the
Númenóreans. We know they had rather long lifespans, and that
Aragorn himself descended from that line. The Númenóreans were "
elf friends". They were the most noble of
men during the
First Age. Genetics 101 dictates that there are dominate and recessive traits than can be passed generation to generation. If, given the above point regarding Arwen and her father and uncle (half-bloods who maintained immortality), we assume that this implies the mortality is a
dominate gene, then it stands to reason that the Númenóreans, who were friends with the elves, likely intermarried a good deal. Those mixed marriages led to half-bloods choosing mortality then carrying on through the generations that
dominate gene, gifting them with long life. The Dúnedain were the end result of this - a group of people who carried many of the positive elven traits (long life, good health, so forth), though lacking in the inbreeding that the Elves of the Second and Third
Ages were so highly guilty of.
That... yes, I tend to ramble, was a long-winded commentary on my perspective on elven genetics and how it impacts them as a race. The point I was getting to is that when considering the above, I don't think it unreasonable to conclude that without the immortality holding genetic deformity (physically, mentally, and emotionally) at bay, Elves probably would have died out or mutated into
monsters long before the
events of
Lord of the Rings ever happened. I also believe this was a contributing factor to why the Elves joined forces with the mortals of Middle Earth when dark forces were rising - perfectly aware that dark forces would nix their immorality and leave them falling to literal bits. It was self preservation that got them involved in any of it.
Just my two cents... forgive me if that was not quite "on topic", but Starflower did comment on the genetics and it is something that I really do believe is important to consider in any discussion about Elves.
-Fir
