Fluorescence polymorphism frequencies have been determined for a group of 85 Down's syndrome cases and 164 controls. For one class of polymorphism, that of positive satellites of chromosome 21, the frequency in the Down's cases was significantly higher than in the controls; the distribution of positive satellites in the mongols indicates that in the majority the extra chromosome arose by first meiotic non-disjunction. The possibility that positive satellites on chromosome 21 could be a causative factor in Down's syndrome is discussed, and the implications of this possibility on the assessment of the risk of producing a Down's child are examined.
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