Inbreeding used to be common Sephardic Jews (Jews from Arabic lands). Some communities, like the Syrian Jews, are quite insular. According to an acquaintance, a social worker, the Sephardim have high rates of mental retardation. Ashkenazic Jews (like Yours Truly) are also a genetic time bomb as far as certain diseases like Tay Sachs or cystic fibrosis.
Fortunately the situation is changing, as Jews are losing their provincialism and there is greater "intramarriage" (Ashkenazim marrying Sephardim; absorption of converts.) Also, many religious Jews go for genetic screening before they begin to date (and most of these marriages are arranged).