http://en.wikipedia.org/wiki/Miscegenation

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Wikipedia also has an article in inbreeding depression, which discusses inbreeding among humans:

http://en.wikipedia.org/wiki/Inbreeding_depression#In_humans

In humans

Although severe inbreeding depression in humans seems to be highly uncommon and not widely known, there have been several cases of apparent forms of inbreeding depression in human populations. As with animals, this phenomenon tends to occur in isolated, rural populations that are cut off to some degree from other areas of civilization. Some notable examples include:

The Vadoma tribe of western Zimbabwe, many of whom carry the trait of having only two toes due to a small gene pool.

The Ulas family of southern Turkey have several members who walk on all fours, due to mental retardation caused by inbreeding.