THE INCIDENCE of Down's syndrome (mongolism) is well known to be correlated with advancing age of the mother: the older the mother is at the time of conception, the greater the likelihood that the child will be a mongol.1,2 The frequency-age distribution curve, however, is bimodal, ie, it may be the resultant of two superimposed curves.3,4 Penrose5 postulated that these curves represent two groups of progeny. The majority (about three quarters) arises from an aging effect in the mother that leads to meiotic nondisjunction and to eventual inheritance of a "free" extra 21 chromosome.
Penrose also suggested that the minority of mongols (about a quarter) result from age-independent factors. This group is perhaps of greater interest because a variety of chromosomal mechanisms have already been implicated as casual factors. In addition the parents are generally in their early reproductive period and are anxious to obtain a meaningful