Familial Robertsonian translocation 15;21 and rare paracentric inv(21): unexpected re-inversion in a child with translocation trisomy 21

We present a family with a Robertsonian translocation (BT) 15;21 and an inv (21)(q21.1q22.1) which was ascertained after the birth of a child with Down syndrome. Karyotyping revealed a translocation trisomy 21 in the patient, The mother was a carrier of a paternally inherited RT 15;21. Additionally, she and her mother showed a rare paracentric inversion of chromosome 21 whi ch could not be observed in the Down syndrome patient. Thus, we concluded t hat the two free chromosomes 21 in the patient were of paternal origin. Rem arkably short tandem repeat (STR) typing revealed that the proband showed o ne paternal allele but two maternal alleles, indicating a maternal origin o f the supernumerary chromosome 21. Due to the fact that chromosome analysis showed structurally normal chromosomes 21, a re-inversion of the free mate rnally inherited chromosome 21 must have occurred. Re-inversion and meiotic segregation error may have been co-incidental but unrelated events. Altern atively, the inversion or RT could have predisposed to maternal non-disjunc tion.