Activity of rRNA genes in cells of a patient with Down syndrome mosaic.

Disomic and trisomic cells of a patient with Down syndrome mosaic were used to study the effect of the additional chromosome 21 against an identical genetic background. The frequency of AG staining and the participation in satellite associations were determined for each pair of acrocentric chromosomes. The additional chromosome 21 of the trisomic cells and its homologues proved to be regularly Ag positive. Therefore the trisomic cells showed more AG positive chromosomes and more satellite associations per cell than the diploid cells. Thus, no compensation for the additional rRNA-gene dose could be found in the cells of the trisomic line.