<prompt> Down syndrome is due to non-disjunction of -21181115  <response>: Ans. is a i.e., 21 Chromosome In 95 of cases of Down syndrome-trisomy of 21:-o Extra chromosome is of maternal in origin.o 1 have mosaic with some all have 46 chromosome,o 4 have robertsonian translocation,o t 13 : 21o t 14 : 21o t 15 : 21o Very rarely long arm of chromosome 21 is triplicate Partial trisomy. 