Genetic disorders regarded as "Jewish" cannot be ruled out in mixed relationships, a world authority on Tay-Sachs has explained.

"These diseases are no respecters of race or religion," says Professor Tim Cox, director of research and honorary consultant physician at University of Cambridge and leading researcher on Tay-Sachs treatment. "They affect people all over the world."

The conditions were over-represented among Jews because Ashkenazi communities had reproduced from a limited gene pool.

"If you live in a small community anywhere in the world, you have a very high chance of genetic diseases that are recessively inherited.

"Each of us carries about 100 mutant genes for potentially lethal diseases. But if you come from a small community, you might share common ancestry, so some mutant genes could be shared. Diseases anywhere in the world will increase in small communities," Prof Cox explains.

He adds that although Tay-Sachs has been largely eliminated among Jews because of heightened awareness and screening, cases still occur.

Prof Cox relates the stories of two non-Jewish couples, both parents of young children who are dying of Tay-Sachs. The disease can go undetected when couples might not know of their Jewish ancestry.

"Sometimes disasters occur. You cannot conclude that marrying out is the answer, because it happens everywhere."