<prompt> Downs syndrome is most commonly caused by ?Maternal nondisjunctionPaternal nondisjunctionTranslocationMosaicism  <response>: Ans. is. a i.e., Maternal nondisjunction o The most common cause of trisomy 21 Downs syndrome is maternal nondisjunction in meiosis 1. In most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum and in 95 of the cases the extra chromosome is of maternal origin. 