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
Down's syndrome occurs due to nondisjunction of which chromosome? 
Select and explain. 
A. chr 13, B. chr 15, C. chr 18, D. chr 21
D: chr 21
Answer: D: chr 21 
Explanation: Ans. is 'd' i.e., chr 21 * In 95% of cases of Down Syndrome - trisomy of 21. Extra chromosome is of maternal in origin* 1% have mosaic with 46 chromosomes.* 4% have robertsonian translocation.# t (22 : 21)# t (14 : 21)# t (15 : 21)* Very rarely long arm of chromosome 21 is triplicate (Partial trisomy)Other trisomies* Trisomy 13 - Patau Syndrome* Trisomy 18 - Edward Syndrome* Trisomy 21 - Down Syndrome