rDNA and acrocentric chromosomes in man

DNA extracted from human subjects who are carriers of different numbers of nucleolar organizer regions (NOR) was hybridized with 3H-rRNA. The subjects with 21 and 13 trisomy, carriers of 11 NOR, show an increase of 10% in the amount of DNA complementary to rRNA; the mongol patients with 14/21 or 21/22 translocation show levels lower than normal; phenotypically normal subjects, carriers of D/D or D/G translocation, show a decrease of 20% in the amount of rRNA bound per unit DNA. The experiments reported confirm that all acrocentric chromosomes carry NOR and that all rDNA is located on these chromosomes. It is probable that the number of rRNA genes, although a polymorphic character, is an inheritable attribute of given nucleolar organizer.