RESUMEN
Based on selective silver nitrate staining of active ribosomal gene (AcRG) clusters in nucleolus organizer regions (NORs) of human metaphase chromosomes, a technique was developed earlier to estimate the AcRG dosage in individual genomes as a sum of arbitrary units (0-3) ascribed to the silver precipitate (AgNOR) on ten NORs. The AcRG dosage was considered to be an additive quantitative trait determined by five polymorphic autosomal loci (with for allelic forms for each locus). A database was created to contain the data on AcRG cluster variants for more than 1000 individual human genomes. In this study, the population frequencies of AcRG cluster variants were determined. The results agreed with the hypothesis that stabilizing selection acts at the zygotic and/or early embryogenetic stage to restrain the AcRG genomic dosage (copy number) within a range from 14.9 to 23.7 arbitrary units (the cell is unviable when the trait is beyond this range). The average zygotic losses due to selection were estimated at 9.1-9.9% for a real population. A computer model where the AcRG dosage of a progeny results from a random combination of the AgNORs of the five acrocentric chromosome pairs of the parents was developed and used to simulate the formation of a certain AcRG genomic dosage through generations in a human panmictic population with nonoverlapping generations. A combination of stabilizing selection by total AcRG copy number and a certain spontaneous mutation rate (the probability of changes in the cluster size of a NOR as a result of unequal crossingover in meiotic prophase) was shown to be a sufficient condition for the restrain of equilibrium population frequencies of AgNOR size variants in a human panmictic population. Using the model, the most probable spontaneous mutation frequency was predicted to be (2.1-2.3) x 10(-2) per NOR per generation for human AgNORs. The predicted frequency was within the 95% confidence interval of the experimental rate, which was determined by studying the inheritance of AgNOR variants in real families.