A Down syndrome child is mentally retarded. All cytogenetic abnormalities may occur except?
Deleted chromosome 21
Trisomy 21
Robertsonian translocation
Mosaic
a. Deleted chromosome 21(Ref: Nelson's 20/e p 610-615, Ghai 8/e p 637-639)An extra copy of chr 21 is required to cause trisomy 21 (Down syndrome), so deletion of chr 21 will not cause it.
Genetics And Genetic Disorders