<prompt> Downs syndrome occurs due to nondisjunction of which chromosome?chr 13chr 15chr 18chr 21  <response>: 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 trisomyOther trisomies Trisomy 13 - Patau Syndrome Trisomy 18 - Edward Syndrome Trisomy 21 - Down Syndrome 