(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.