Down's syndrome is genetic disorder caused due to the presence of an additional copy of chromosome 21 - trisomy of 21. The symptoms are: the affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.
This is caused due to the non disjunction of 21* chromosome during oogenesis. Egg would have 24 chromosomes instead of 23 chromosomes. Non - disjunction increases with maternal age because ova developed in female since birth and older cells become more prone to such abnormalities due to various physico - chemical exposures.