A cytogenetic survey of the complete population of mentally subnormal in the North-East of Scotland has been undertaken. A register for the mentally subnormal within the region already existed, and all persons recorded, whether they resided at home or in subnormality hospitals or other institutional care, were included. The total number recorded was 3020 and of these 2770 were examined cytologically. In all 297 (10.7%) were shown to have a chromosomal abnormality, and of these Down's syndrome accounted for 250 (9%). Within this category was an unexpected excess of males. Deletions and supernumeraries comprised the remaining autosomal anomalies. Increased numbers of sex chromosome abnormalities among high grade mentally subnormal individuals were confirmed for both sexes. The survey has shown that abnormal chromosome complements contribute significantly to the causation of mental retardation, and has also provided estimates which cannot be obtained from hospital surveys alone.
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