Data on nonspecific mental retardation in two or more sibs ascertained from the British Columbia Health Surveillance Registry are utilized to calculate the frequency of X-linked mental retardation in the population of British Columbia (B.C.). In the 1950-1969 birth cohort there are 107 sibships with only males affected and 35 sibships with only females affected giving an excess of 72 male-affected sibships. Family histories of the sibships with two more more mentally retarded males provide evidence that the mothers of the excess of male-affected sibships are carriers of X-linked mental retardation. Thus, a frequency of 1.83 per 1,000 males for X-linked mental retardation and carrier frequency of 2.44 per 1,000 females are calculated. This frequency can account for all the excess of males in the population who are affected with nonspecific mental retardation. We estimate that seven to 19 X-linked genes exist to cause nonspecific mental retardation, and that the frequency of the marker-X chromosome, associated with X-linked mental retardation in some families, may be 0.92 per 1,000 male births.

Seven families with X-linked mental retardation (MR) have been studied clinically and cytogenetically. All affected males in six of the families were found to have a fragile site on Xq in a number of their peripheral lymphocytes. The fragile site was not seen in any of the affected males in the seventh family. The affected males in the six families with the fragile X had a syndrome characterized by a variable degree of MR, macro-orchidism, a characteristic repetitive, jocular speech, normal body proportions, and large jaws and ears. The fragile X chromosome could only be detected in a proportion of female carriers and its frequency in females was found to be correlated with their mental status to be inversely correlated with their age.

Chromosomal, clinical, and psychological data are presented on members of six families with X-linked mental retardation. Affected males in three of these families express the fra(X)(q28) marker, while the retarded males in the other three do not. Similar variable physical and psychological characteristics, such a lop ears, large testes, and perseverative speech, are present in affected males in all six families. Preliminary analysis of the psychological data also shows that males with and without marker expression cannot be differentiated with certainty. On this basis we suggest that there is a type of X-linked mental retardation with many phenotypic features of marker-X mental retardation but without expression of the X chromosome fragile site.

Cases of nonspecific mental retardation (MR) born in British Columbia between 1952 and 1970 ascertained through the British Columbia Health Surveillance Registry were linked by birth registration number to family sibships from computer-linked groupings of birth and marriage records in British Columbia. It was possible to retrieve family information for 97% of the cases by this method. Because of good ascertainment and relatively large sample size, the 1952-1965 birth cohort comprising 2,209 index cases was selected for calculations of overall risks and recurrence risks to sibs categorized by sex, MR level, associated neurological disability, and singleton versus multiple birth. The overall risk of affected individuals among all sibs was 4.4 +/- 0.6%, which was about ten times greater than the minimum population incidence of nonspecific MR. The risk among subsequent sibs of the first affected case in a family was 3.7 +/- 0.8%. These risks varied depending on sex, MR level, and whether the mental retardation was associated with hydrocephalus, microcephalus, cerebral palsy, or epilepsy. The recurrence risk after two affected individuals was 12 +/- 7%--about three times greater than after one affected individual. Even though the frequency of MR is greater among twins than in the overall population, the recurrence risk of nonspecific MR was not significantly different for index cases from either singleton or multiple births.

Classical infantile autism occurs more frequently in males and has recently been noted in patients with the fragile (X) form of X-linked mental retardation (XLMR). In order to better understand this association and to determine whether fra(X) XLMR could account for the excess of autistic males, we investigated a group of institutionalized severely handicapped adults, 33 males and eight females, who were diagnosed as autistic using the DSM III diagnostic criteria of infantile autism. Chromosome studies using FUdR showed that three of the males had the Xq27 fragile site. We confirmed the association of autism and fra(X) XLMR, and showed that this extreme form of behaviour is part of the spectrum seen in the Martin-Bell syndrome. Two of the three autistic males with the Xq27 fragile site had a history of birth insults, which in combination with developmental deficits due to the fragile X gene, might have led to the behavioural disorder. Even though the fragile X cannot account for the excess of males with classical autism, it is an important X-linked factor in its cause. The diagnosis can allow more accurate counselling for this subset of autistic males.