Down syndrome is due to non-disjunction of -

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 : 21)o t (14 : 21)o t (15 : 21)o Very rarely long arm of chromosome 21 is triplicate (Partial trisomy).