M.A. Ferguson-Smith

Nucleolus-organiser regions in familial extra metacentric human chromosomes

SummaryA small extra bisatellited metacentric chromosome has been found in two unrelated normal females and their foetuses. The presence of two nucleolus-organiser regions (NORs) on these chromosomes was defined by in situ hybridisation of 28S and 18S ribosomal genes, silver staining and the observation of satellite association. C-banding strongly suggests that they are also dicentric.The characteristic silver-staining and the high frequency of involvement of the marker chromosome in satellite association indicates the functional activity of the nucleolar-organiser regions on both extra chromosomes. In both cases, it seems likely that the extra chromosome has arisen by a centric fusion translocation and is not an isochromosome, because the chromosome arms show differences in length, satellite size, Ag-staining and participation in satellite association.