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What is A Down syndrome child is mentally retarded. All cytogenetic abnormalities may occur except? and explain in detail?
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.