<prompt> Which mutation is seen in Downs syndrome -Monosomy XTrisomy 21Trisomy 22Trisomy 13  <response>: Ans. is b i.e., Trisomy 21 o Basic molecular defect in Downs syndrome is extragenetic material of chromosome 21. It may be due to :1 Trisomy 21 This is the most common cause 95. There is an extra chromosome 21 i.e. three 21 chromosomes which is due to meiotic nondisjunction in ovum. The extra chromosome is of maternal origin. The most important risk factor is advanced maternal age 35 years.2 Roberston translocation This accounts for 3 of cases. Extra material of 21 chromosome comes from Robertson tanslocation of long arm of chromosome 21 to another acrocentric chromosome 22, or 14 or 15, i.e. t 22: 21, t 14:21, or t 15:21. It has no relation with maternal age.3 Mosaicism Mosaic 21 In 2 of patients there is mosaicism, i.e. occurance of two or more different types of cell population 46 XX 47 XX 21 in same individual, due to mitotic nondisjunction of chromosome 21. There is no correlation withmaternal age.4 Partial trisomy It is very-very rare. The long arm of chromosome 21 triplicates. 