At risk for Down's syndrome.

Some families have a hereditary characteristic that predisposes them to Down's syndrome, report geneticists at Johns Hopkins Medical Institutions in Baltimore. This factor is unrelated to the mother's age. Although much previous work has documented that the risk of Down's syndrome increases with the age of the mother, Stylianos E. Antonarakis and his colleagues observed that in a Greek population there was the same frequency of the Down's syndrome births among mothers of various ages.

Antonarakis found that Greek families in which Down's syndrome had occurred were twice as likely as other Greek families to have a particular version of chromosom number 21, the chromosome that is present in an extra copy in the cells of individuals with Down's syndrome. He identified the chromosome by a set of variations in DNA sequence called polymorphisms. This version of chromosome 21 is more likely to fail to divide normally during cell reproduction, Antonarakis says. Geneticists are now investigating whether polymorphisms are associated with a predisposition to having a child with Down's syndrome in other ethnic groups.