But Risch's team found that mutations responsible for Tay-Sachs and other lysosomal storage diseases — which involve an accumulation of fatty substances in enzyme-containing cells — are no more common than for other inherited diseases and are the result of chance.

Using advanced molecular methods, Risch's team studied the DNA around mutations to determine that most of those mutations date back some 50 generations to around the year 900 C.E. and the establishment of the Ashkenazi community.

"This is what happens when you have basically a small population," said Risch. When such groups remain homogenous, they end up with a "distinct pattern of mutations," he said.

His findings were published this spring in the American Journal of Human Genetics.

— Abby Cohn