STRUCTURAL VARIABILITY OF HUMAN CHROMOSOME-9 IN RELATION TO ITS EVOLUTION

Human chromosome 9 shows a high susceptibility for structural rearrangements, particularly pericentric inversions, which often are transmitted. Three types of pericentric inversions can be observed on No. 9: Type I, showing the total constitutive heterochromatin in the short arm; Type II with part of the C heterochromatin on the short arm, the rest located on the long arm proximal to the centromere; Type III, a subtelocentric chromosome with part of the C heterochromatin in the very short arm and the rest located interstitially on the long arm. With these inversions and other structural rearrangements, e.g., translocations, the break-points are located preferentially within the C heterochromatin or close to the heterochromatic-euchromatic junctions. These contrast the findings in lymphocytes from 5 patients with Fanconi''s anemia and those after irradiation in vitro, reported in the literature. In lymphocytes break-points seem to be distributed more or less by chance. Human chromosome 9 was primarily an acrocentric chromosome; in morphology and at least in some functions similar to D- and G-group chromosomes. During evolution this acrocentric chromosome changed to a submetacentric one due to a pericentric inversion.