A down syndrome child is mentally retarded. All cytogenetic abnormalities may occurs except ?

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.