Consanguinity, non-disjunction, parental age and Down's syndrome.

The effect of consanguinity on the non-disjunction mechanism in Down's syndrome, was examined in 417 cytogenetically confirmed Down's syndrome patients. The incidence of parental and grandparental consanguinity was 17.5% (n = 73), while that of only parental consanguinity was 17.2% (n = 71). First cousin marriages occurred more frequently, than uncle-niece and 2nd cousin marriages. With regard to parental age, only the mean age difference between consanguineous couples significantly differed from non-consanguineous couples. The inbreeding coefficient did not differ between trisomy 21 and translocation Down's families. From this study, it is concluded that consanguinity does not predispose to Down's syndrome.