For instance Tay Sachs disease, Canavan disease and a specific form of cystic fibrosis are examples of diseases which appear at higher frequency in the Ashkenazi Jewish population. All three are lethal diseases for which genetic screening tests are now available. Screening tests allow prospective parents to determine whether they are carriers for genes. Mutations can occur in any cell of the body, including the sperm and eggs. And since many of these disorders are incurable, prevention is the most important tool.
Prenatal diagnosis can detect fetuses with lethal genetic disorders very early in a pregnancy.
Screening programs such as the one at Hadassah Hospital have reduced the number of Israeli babies born with cystic fibrosis and Tay Sachs to almost negligible numbers. Even Ultra-Orthodox, or Haredi, Jews in Israel participate in screening for genetic diseases. However, since Haredi rabbis usually do not permit abortions, in that community they do not wait for screening until a couple conceives. They do not even wait for a couple to meet. In the Haredi, community there is a program called Dor Yesharim, where couples are pre-screened before meeting.