It is Gods will. The Jews have been breeding and producing children with their sisters and cousins for centuries in order to keep their blood Jewish. God frowns upon such practices and has cursed their blood.
Individuals of Ashkenazi Jewish descent are at an increased risk for certain autosomal recessive genetic disorders. These disorders include cystic fibrosis, Canavan disease, familial dysautonomia, Tay-Sachs disease, Fanconi anemia, Niemann-Pick disease, Bloom syndrome, mucolipidosis type IV, and Gaucher disease, among others.
It is estimated that one in every four or five individuals of Ashkenazi Jewish descent is a carrier for one of these conditions.
Due to the increased incidence of these diseases in this population (see the table in the Ashkenazi Jewish Genetic Diseases Test Fact Sheet), routine preconception or prenatal expanded carrier screening is recommended.