Satellite DNA and cytological staining patterns in heterochromatic inversions of human chromosome 9

A series of partial inversions of the heterochromatic C-band of chromosome 9 have been stained with distamycin A plus 4',6-diamidino-2-phenyl-indol-2HCl (DA/DAPI) and found to consist of three classes: (a) those in which only the C-band in the long arm fluoresces with DA/DAPI (these are the most frequent), (b) those in which only the C-band in the short arm fluoresces with DA/DAPI, and (c) those in which the C-bands in both arms fluoresce with DA/DAPI. There are also differences in the satellite DNA content of each type of inversion as measured by hybridisation in situ. Types (a) and (b) have satellite DNA contents similar to those of their normal homologues, which type (c) has a satellite DNA content almost double that of the normal homologue. It appears that DA/DAPI specifically stains heterochromatin that contains satellite DNA. The ability to distinguish these three types of inversion may help to resolve the question of the clinical significance of such inversions.