The paper presents results of a revision of data of both conventional chromosome testing and a study of cytogenetic (QFQ) markers in families with Down syndrome. Retrospective analysis of 151 families found eight families with a carrier of gonadal mosaicism. In all cases, the mother was younger than 35 years old. Therefore a prevalence of parental mosaicism in young couples was estimated to be 6,5 % (8/123). Conventional diagnostic testing, not followed by analysis of segregation of QHQ markers, would have resulted in a prevalence of only 1 %. A comparison of the results of cytogenetic analysis with those expected using molecular polymorphisms suggests that cytogenetic testing cannot be entirely replaced by molecular testing. A combination of both methods should be applied when gonadal mosaicism is suspected.
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