<prompt> A Down syndrome child is mentally retarded. All cytogenetic abnormalities may occur except?Deleted chromosome 21Trisomy 21Robertsonian translocationMosaic  <response>: a. Deleted chromosome 21Ref: Nelsons 20e p 610-615, Ghai 8e p 637-639An extra copy of chr 21 is required to cause trisomy 21 Down syndrome, so deletion of chr 21 will not cause it. 