Thanks for the quote by Tolkien, yet he tried to give explanations for his whole sub-creation, if he knew genetics he probably would used it.

Also to BirdofHermes:

Race were are talking genetic traits, we were talking about the broad definition all along, so yes race=species in a broad sense.


For the rest, check this picture made by a fan of tolkien about half-elf genetics:
http://i24.photobucket.com/albums/c1.../halfelven.jpg