The role of cytologic NOR variants in the etiology of trisomy 21.

Silver-stained chromosomes from 29 couples with a trisomy 21 offspring and from 25 control couples were studied to determine whether there was an association of nucleolar-organizing-region variants in parents of children with trisomy 21. A reproducible scoring system for the analysis of silver-stained chromosomes was developed, and this was applied to the analysis of study participants in a blinded fashion. Seven of the 58 parents of children with trisomy 21 and seven of the 50 control parents were found to have variant NORs on silver staining. Therefore, we do not find a demonstrable risk for nondisjunction of chromosome 21 in individuals with silver-staining variants.