Down's syndrome occurs due to nondisjunction of which chromosome?

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