Tay-Sachs disease is an untreatable disorder that is always fatal. Children born with Tay-Sachs appear normal at birth but lack an enzyme needed to break down certain fats. The fats build up and destroy brain and nerve cells, leading to rapid mental and physical deterioration and death in early childhood.
Approximately 1 in 27 Ashkenazi Jews is a carrier of Tay-Sachs disease. The rate for French Canadians and Cajuns is similarly high, and approximately 1 in 50 Irish Americans is a carrier.
Carrier screening will detect 97-99% of Ashkenazi carriers.
More information on Tay-Sachs disease from the Mount Sinai School of Medicine.