why only 21st chromosome get disturbed in Down's syndrome

Down’s Syndrome causes due to the presence of an additional copy of chromosome 21 (Trisomy of 21).  The trisomy means occurrence of three copies of the chromosome number 21, which occurs by non disjunction of chromosomes during gametic meiosis. An individual with Down's syndrome is characterised by webbed neck, short stature, mongoloid face, protruding tongue, bulging eyes, mental retardation and are short lived.

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