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.
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