Inbreeding... and a little incest

[Elladan and Elrohir]

Alatar, I believe The Silmarillion says that Aule made seven dwarves. In one of Tolkien's letters, he says that there were thirteen originally, seven males each with his mate, "save Durin the eldest who had none." This latter account seems far more likely to me, since if there were only seven to begin with, they would have to be of mixed gender, and it's difficult to see how the Seven Houses of the Dwarves could have arisen from that.