Fathers and mothers of Down's syndrome cases show dermal microsymptoms when a large series of parents are compared to the general population. A Walker dermal index score in the overlap range (-2·99 to +3·00) is more likely to occur in fathers of age-dependent Down's syndrome cases (mean paternal age 40, range 25 to 54 years) and in Down's syndrome mothers than in the general population. The relative risk for these fathers to have a dermal index in the overlap range is two times the risk for male controls; the corresponding risk for mothers of Down's syndrome cases is 1·6 times that for female controls. Thus a score in the overlap range may be used to indicate a group of parents at higher risk for recurrence and occurrence of trisomy 21 offspring. This higher risk parent group can be offered cytogenetic studies, including amniocentesis and chromosome analysis on peripheral blood and skin, as dictated by clinical circumstances.
From a comparison of dermal indexes in these studies, the contribution of maternal mosaicism to all cases of Down's syndrome is estimated to be about 11% and the contribution of paternal mosaicism about 8%. The contribution from mosaicism in the father but not in the mother appears to increase with parental age. To confirm these estimates, more parents with trisomy 21 mosaicism and trisomy 21 offspring must be diagnosed and studied quantitatively for dermal microsymptoms.
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