Familial silver staining patterns of human nucleolus organizer regions (NORs)

The silver staining patterns of the nucleolus organizer regions (NORs), an indication of rDNA transcriptional activity, were studied in metaphases from lymphocyte cultures of 20 karyotypically normal members of three families selected for a large sibling number or a monozygotic twin pair. Quinacrine polymorphic markers and bands were used to identify the acrocentrics and to determine their parental origin. A comparison of the silver staining frequencies among siblings and between parent and child indicated no significant differences for any acrocentric in the twin pairs and significant differences (P less than .05) for only one of the 20 acrocentrics segregating in each of two families. These two acrocentrics had short stalks with very small silver deposits (AgNORs). The mean size of the AgNOR, based on a relative score, was not significantly different (P greater than .05) for each homolog between the twin pair and in approximately 70% of the acrocentrics shared by members of the one family analyzed. The frequency with which a particular chromosome was silver stained demonstrated a significant correlation (r2 = .732) with the size of AgNOR. There was a close correlation (r2 = .609) between stalk length and the size of the AgNOR. We conclude that the frequency of silver staining and the mean size of the AgNOR are characteristics inherent in a particular chromosome carried from one generation to the next.