The genetic control of metabolic processes and the close association between certain metabolic and mental defects have recently been reviewed.1,2 Within an institution for the mentally defective, however, only 2% to 3% of the patients have known biochemical abnormalities which account for their defects, while some 60% to 70% are clinically diagnosed as "familial" or "undifferentiated," indicating that the etiology of the mental defect is unknown.
Many genetically determined biochemical abnormalities associated with mental defect have been first described in siblings. There were two defective brothers among the first group of patients with phenylketonuria described by Fölling3; Baron and co-workers described Hartnup disease in two sibs in one family4; Allan recently described aminoaciduria with mental deficiency in a brother and a sister5; "maple syrup" urine disease was first noted in four sibs in one family,6 and Bigler recently described an abnormality of lipoproteins in two
WRIGHT SW, TARJAN G, EYER L. Investigation of Families with Two or More Mentally Defective Siblings: Clinical Observations. AMA Am J Dis Child. 1959;97(4):445–463. doi:https://doi.org/10.1001/archpedi.1959.02070010447010
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