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