Inbreeding

Global prevalence of consanguinity

"As a working definition, unions contracted between persons biologically related as second cousins (F ³ 0.0156) are categorized as consanguineous. This arbitrary limit has been chosen because the genetic influence in marriages between couples related to a lesser degree would usually be expected to differ only slightly from that observed in the general population."
(Background summary of consanguinity)

As you can see, consanguineous marriages are low among most Europeans and European descendants, and comparatively high among groups from most other parts of the world that have been investigated (possibly excluding some northern Mongoloids). Consanguineous marriages are especially high in the Muslim world, and even among non-Muslim Middle Easterners and their descendants.

Jewish inbreeding

Jewish groups, including Ashkenazi Jews, are heavily inbred. This inbreeding results both from the relatively small initial size of the Jewish gene pool coupled with strong tendencies towards endogamy, as well as the fact that Jewish culture traditionally allows and favors highly consanguineous marriages. Judaism allows uncle-niece marriages, for example.

Religious law may differ however, and marriage between uncle/niece and aunt/nephew is specifically forbidden in the Koran but permitted for Jewish communities.
http://www.genetics.com.au/genetics/resource/factsht/F13.pdf

Jewish: Uncle-niece and first-cousin marriage is allowed.
http://www.geneletter.org/features/issue2/kissingcousins.html

For example, first cousin marriages are legal in countries such as the U.K. and Australia, but they are criminal offences in eight of the states of the U.S.A. and illegal in a further 31 states (Ottenheimer 1990). Yet exceptions can be incorporated into state laws, for example, to permit uncle-niece marriage within the Jewish community of Rhode Island (Bratt 1984).
http://www.consang.net/Summary/

The co-efficient of inbreeding for an uncle-niece marriage is 1/4, compared to 1/2 for parent-child and 1/8 for first cousins. Among (non-Jewish) white Americans, even first cousin marriages came to be seen as backwards. It is amusing when certain Jews call white individuals "inbred" without realizing that they are likely to be much more inbred than the person they're applying that label to; and without realizing that their Jewish culture sanctions highly consanguineous marriages that many white Americans would find disturbing. By using "inbred" as an insult, they are ignoring their own culture and background and adopting mainstream American snobbery, which snobbery should deem real Jewish practices even worse than those of caricatured "rednecks".

An individual afflicted with Bloom Syndrome.

The effects of Jewish inbreeding

Jewish inbreeding has led to the emergence of various "Jewish diseases". Diseases unusually common among Ashkenazi Jews include Tay-Sachs, Gaucher, Canavan, certain breast cancer genes, Mucolipidosis IV, and others. Bloom Syndrome, for example, is caused by a problem in a gene that:

is involved in the copying and repair of the genetic information in the cells of the body. The most serious characteristics of this condition are a predisposition to cancer and infections.

Other features of Bloom Syndrome:

• Short stature
• Immunodeficiency
• Photosensitivity
• Diabetes mellitus
• Male sterility