Ridley dwells at some length on the issues raised by foreknowledge. He points with some admiration to the efforts of Jewish Americans to protect their progeny from cystic fibrosis and Tay-Sachs disease, both of which are consequences of specific mutations of the BRCA2 gene, located on chromosome 13. In the United States, the Committee for the Prevention of Jewish Diseases organizes the testing of schoolchildren's blood. When matchmakers later consider potential marriages, they can call a hotline and quote the anonymous number assigned to each child at the time of testing. If both potential partners are carriers of the same mutation, be it for cystic fibrosis or Tay-Sachs disease, the committee advises against the marriage.
Compliance is purely voluntary.
Tay-Sachs disease is becoming increasingly rare in the U.S., and cystic fibrosis has been all but eliminated from the American Jewish population. Genes, according to Ridley, determine aspects of behavior as well as health. Mating behavior is perhaps the most obvious example. Although no one yet claims to know how such messages are transmitted, Ridley expresses confidence that a definitive answer will one day be found. Right or wrong on this point, his book is an artful blend of vital information and informed speculation.