A subtelocentric chromosome 9 in a dysplastic 18-year-old boy with dissociated mental development.

A subtelocentric C-group chromosome was identified as inv(9) (p24q12) in an 18-year-old young man who shows few dysplastic signs and who suffers from dissociated mental development. Pericentric inversions producing an almost metacentri No. 9 have been reported in several studies. The frequency of which proved to be high. There is an apparent lack, however, of inverted acro- or subtelocentric No. 9 reported in the literature. This obvious difference in the break points on chromosome 9 is most likely due to 1. hot-spots for chromosome breakage i.e. highly susceptible regions such as the euchromatic-heterochromatic junctions. 2. a higher degree of elimination against such acro- or subtelocentric chromosomes from one generation to the other.