A cytogenetic study of 47,XXY males of known origin and their parents.

A number of patients with both sex chromosome aneuploidy and the fragile X syndrome have been reported and this has led to the hypothesis that females heterozygous for the fragile X mutation have an increased rate of meiotic nondisjunction. Furthermore the suggestion has frequently been made that a predisposition to meiotic nondisjunction is associated with an increase in mitotic nondisjunction. We have tested these two hypotheses by examining the chromosomes of a series of 47,XXY males and their parents. In the majority the parental origin of the additional sex chromosome was known. The cells were cultured under conditions suitable for demonstrating the fragile X and 100 cells were scored 'blind' from the patients and both parents. No fragile X individual was seen and there was no difference in the proportion of aneuploid cells between the parents in whom the nondisjunction event occurred and the control parents. Therefore, our results lend no support to the suggestions that the fragile X is associated with an increase in sex chromosome aneuploidy or that individuals in whom meiotic nondisjunction occurs have an increased level of mitotic nondisjunction.