This term describes a group of conditions which are unusually common among Jews of Ashkenazi, Central and Eastern European, descent. Although these diseases can affect Sephardi Jews and non-Jews, they appear in Ashkenazi Jews more often—as much as 20 to 100 times more frequently.
Scientists believe that certain disorders became more common among Ashkenazi Jews because of at least two processes: the “founder effect” and genetic drift.
Founder effect refers to the chance presence of these genes among the “founders” or ancestors who immigrated to Eastern Europe during the Diaspora (70 C.E.).
Genetic drift refers to the increase in frequency of the genes for these disorders in this group, as a result of chance. Because Jews tend not to marry outside of their faith and community, the relatively high frequency of these genes among Jews did not pass into other communities, nor was the frequency lessened by the introduction of other genes from outside the Ashkenazi population.
Yes, but Sephardi diseases typically differ based on country of origin.