Inbreeding... and a little incest

[Lalwendë]

I've been reading a fair amount about the Amish this week (following the horrible events in Pennsylvania) and I noticed that there is apparently a very high incidence of genetic illness due to the small original gene pool and inbreeding as a result (further exacerbated by not admitting 'outsiders' into the community via marriage). This doesn't result in people who have learning disabilites, more that they have physical disabilities including dwarfism (which Im sure is not the PC term so I apologise), 'short legs' in comparison to the body and polydactylism (extra fingers and toes).

Though of course Tolkien's Dwarves are a race and so very different.

Maybe Tolkien just didn't include the women in that original figure of seven?