Supernumary marker chromosomes in a mentally retarded population identified as inv dup(15).

Bisattelited supernumary chromosomes, identified as inv dup(15)s, were detected at a prevalence of 8/1000 among institutionalized, mentally retarded (non-Down syndrome) males. This finding suggests that the inv dup(15) contributes significantly to the etiology of mental retardation among institutionalized individuals.