Characterization of a familial balanced rec(13) in a child with mild MR and his half-sibling with two structurally rearranged chromosomes 13.

In this report, we describe a 7-year-old child with mild mental retardation, developmental delay, and learning disabilities. His karyotype contained a rearrangement of chromosome 13, which appeared to include a duplication of 13q31-qter and a deletion of 13p12-pter regions. The chromosomal origin of the additional material was confirmed by fluorescence in situ hybridization (FISH) using a whole chromosome painting probe specific for chromosome 13. Family studies showed that his mother carried a balanced inversion of chromosome 13 and that his half-brother carried the balanced pericentric inversion of chromosome 13 from his mother as well as another structural rearrangement involving chromosome 13 presumably from his father. The findings from this study suggested that the proband's abnormal 13 resulted from an unbalanced crossing-over between the normal and maternal inverted chromosome 13.