What is Down's syndrome? Give its symotoms and cause. Why is it that the chances of having a child with Down's syndrome incrreases if the age of the mother exceeds forth years ?

Explanation

Down's syndrome is a human genetic disorder caused due to trisomy of chromosome no. 21. Such individuals are aneuploid and have 47 chromosomes (2n + 1). The symptoms include mental retardation, growth abnormalities, constantly open mouth, dwarfness etc. The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosome of pair 21 during meiotic division in the ovum. The chances of having a child with Down's syndrome increase with the age of the mother (+ 40) because ova are present in females since their birth and, therefore older cells are more prone to chromosomal non-disjunction because of various physico-chemical exposures during the mother's life-time.