Additional evidence for fragile X activity in heterozygous carriers.

The result of a previous study showing an association between mental development and fragile X activity in heterozygous females is given further support by similar investigations of three additional kindreds. The increased frequency of demonstrable fragile X chromosomes in mentally retarded females appears to be due to an increase in the active fragile X while the inactive marker X remains at a similar low frequency in all heterozygotes whether retarded or not. The frequencies of the active fragile X separated the normal and abnormal subjects into two distinct populations. The suggested inverse correlation between the number of lymphocytes with detectable fragile X chromosomes and advancing age can be attributed to ascertainment biases.