<prompt> A down syndrome child is mentally retarded. All cytogenetic abnormalities may occurs except ?Deleted 21Trisomy 21Robesonia translocationMosaic  <response>: In approximately 95 cases down syndrome is due to presence of 3 copies of chromosome 21. Approximately 1 cases with trisomy 21 are mosaics . Another 4 have a translocation that involves chromosome 21. Majority of translocation are fusions at the centromere between chromosome 13,14,15,21, and 22 known as robesonial translocation. Reference : Nelson TB of pediatrics edition 19 pg 402. 