This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics And Genetic Disorders, Subject name: Pediatrics
A Down syndrome child is mentally retarded. All cytogenetic abnormalities may occur except? 
Select and explain. 
A. Deleted chromosome 21, B. Trisomy 21, C. Robertsonian translocation, D. Mosaic
A: Deleted chromosome 21
Answer: A: Deleted chromosome 21 
Explanation: 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.