Down's syndrome is most commonly caused by

TRISOMY 21: DOWN SYNDROME Incidence: 1 in 700 bihs Karyotypes: Trisomy 21 type: 47,XX, +21 Translocation type: 46,XX,der(14;21)(q10;q10),+21 Mosaic type: 46,XX/47,XX, +21 The correlation with maternal age suggests that in most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum. Indeed, in 95% of cases the extra chromosome is of maternal origin. In about 4% of all patients with trisomy 21, the extra chromosomal material is present not as an extra chromo- some but as a translocation of the long arm of chromosome 21 to chromosome 22 or 14. Such cases frequently (but not always) are familial, and the translocated chromosome is inherited from one of the parents, who typically is a carrier of a robesonian translocation. (Robbins Basic Pathology, 9 th edition. page : 238)