question_answer 51)

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

Answer:

                  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 syndrome are (i) Mental retardation (ii) Growth abnormalities (iii) Constantly open mouth (iv) Dwarfness, etc., gonads and genitalia under developed The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosome (a pair 21 during meiotic division. The chances of having a child with Downs 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 fertilization. During this long gap(till meiosis is not completed) egg cells are arrested in prophase I and chromosomes are unpaired. The greater the time they remain upaired greater the chance for unpairing and chromosome non-disjunction.