Polymorphisms of Ag-stained nucleolar organizer regions in man.

Polymorphisms of the NORs as tested by Ag-staining of metaphase G-banded chromosomes were investigated in cultured blood lymphocytes of karyotypically normal individuals from the Moscow population. The study of cell-to-cell variability in the number of Ag-stained NORs carried out on 14 monozygotic twin pairs showed the phenomenon to have some features of real intercellular variation. In 40 unrelated individuals the individual acrocentric chromosomes were compared by the number of Ag-stained NORs, their degree of staining, and their participation in acrocentric association. Chromosome 21 was found to be significantly more active than four others by all the criteria, and chromosome 15 was less active compared with the others by the size of the Ag deposits and the frequency of participation in NOR associations. The frequency distribution of homozygotes and heterozygotes for Ag-stained NORs in the same group of 40 individuals was in accordance with the Hardy-Weinberg law.