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 number 21. Such individuals are aneuploid and have 41 chromosomes, i.e., (2n+1) symptoms of down's sydrome are
(i) Mental retardation
(ii) Growth abnormalities
(iii) Constantly open mouth
(iv) Dwarfness, etc., gonads and genitalia under developed
The reson for the disorder is the non-disjuction (failure to separte) of homologous chromosome (a pair 21 during meiotic division. The chances of having a child with Down's syndrome increases with the age of the mother (+40) because age adversely affects meiotic chromosome behaviour.
Meiosis in the egg cells is not completed, until after fertilisation. During this long gap (till meisosis is not completed) egg cells are arrested in prophase I and chromosomes are unpaired. The greater he time they remain upaired greater the chance for unpairing and chromosome non-disjuction.