A CASE OF APPARENT TRISOMY-21 WITHOUT THE DOWNS-SYNDROME PHENOTYPE

We describe a case of apparent trisomy 21 that does not fulfil the cri teria for the clinical diagnosis of Down's syndrome (DS). Our patient was subjected to karyotype analysis and found to have full, non-mosaic trisomy 21 in both blood lymphocytes and skin fibroblasts, while exam ination of the term placenta, which was performed earlier in the cours e of a different study, had shown mosaicism (73%) for trisomy 21. FISH analysis showed no obvious rearrangement of the DS chromosomal region in any of the chromosomes 21. Molecular analysis using polymorphic ma rkers on chromosome 21 verified the existence of trisomy for the entir e long arm of the chromosome and showed that the origin of the extra c hromosome was maternal and was probably the result of a mitotic error. In contrast with the above, the clinical evaluation using the Jackson checklist of 25 signs failed to establish the diagnosis of DS; We bel ieve that our patient might present mosaicism in other tissues that ar e not available for analysis and can be regarded as an extreme example in the continuous spectrum of karyotype/phenotype associations in mos aic cases.