Answer the following questions related to Down’s syndrome

Answer the following questions related to Down’s syndrome.
(i) When was Down’s syndrome first described?
(ii) What is its frequency in human and does it also occur in other animals?
(iii) How does Down’s syndrome arise?
(iv) Relate it with chromosome dysfunctions.

(i) It was first described in 1866 by J Langdon Down, so it is called Down’s syndrome.
(ii) About 1 in every 750 children exhibits Down’s syndrome throughout the world. It is also seen in chimpanzees and other related primates.
(iii) In humans, it comes as a result of non-disjunction of chromosome 21 during egg formation. The cause of this primary non-disjunction is not known.
(iv) The defect is associated with a particular small portion of chromosome 21. When this particular chromosome segment is present due to translocation in three copies, instead of two, Down’s syndrome occur.