Cytogenetic characterization of an extra structurally abnormal chromosome associated with severe mental retardation: Inv dup (15) (q13)

We have studied an extra structually abnormal chromosome (ESAC) in a 13 yea rs old boy with profound mental, psychomotor and speech retardation, behavi oral problems, seizures and abnormal electroencephalogram. The examination of the bisatellited ESAC with chromosome banding demonstrated that the kary otype was: 47, XY, +inv dup (15) (pter-->q13::q13-tpter). The cytogenetic c haracterization of the inv dup (15) is reported with special emphasis on th e usefulness of DA/DAPI staining when G-banding is sequentially performed t o discard possible heteromorphisms in DA/DAPI positive chromosomes, and the importance of Ag-NOR heteromorphisms to ascertain the maternal origin of t he inv dup (15). A U-type exchange between two non-sister chromatids is pro posed as its mechanism of formation. The clinical features of the case were consistent with those previously reported in similar cases.