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

(N/A) Down's syndrome is a human genetic disorder caused by the trisomy of chromosome number $21$. Such individuals are aneuploid and possess $47$ chromosomes $(2n+1)$.
Symptoms include mental retardation,stunted physical growth,a constantly open mouth,a furrowed tongue,and short stature.
The cause of this disorder is the non-disjunction (failure to separate) of the homologous chromosomes 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 (above $40$ years) because all ova are present in the female from birth. As the mother ages,these older cells are more prone to chromosomal non-disjunction due to the cumulative effect of various physico-chemical exposures throughout the mother's lifetime.