FAMILIES with two or more mentally retarded sibs represent a group in whom there is an increased likelihood that genetic factors, including chromosome abnormalities, are related to the mental defect. Sibships in which two individuals were identified with different chromosome abnormalities have been reviewed by Hecht et al.1 These include Down's syndrome and trisomy 18 or trisomy D1; Down's syndrome and Klinefelter's syndrome (XXY); Turner's syndrome (XO) and trisomy D1, etc. Sibships in which mental defects and congenital malformations associated with the same chromosome abnormalities have also been described; these include identical schizoid twins with five G-group members,2 two sisters with five G-group members,3 two sibs with mental retardation, five G-group chromosomes and a suggested D/E translocation in the mother,4 and two retarded sibs with trisomy E.5
Thus, the screening of a defined population of families with two or more defective sibs
Wright SW, Day RW, Muller H, Quigley M. Investigations of Families With Two or More Mentally Defective Siblings: III. Chromosome Studies. Am J Dis Child. 1967;114(1):69–71. doi:10.1001/archpedi.1967.02090220075013
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