Which mutation is seen in Down's syndrome -

Ans. is 'b' i.e., Trisomy 21 o Basic molecular defect in Down's 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.