Multiple choice Down's syndrome is due to chromosome number increase in 21st pair autosome chromosome number decrease in 21st pair autosome chromosome number increase in 18th pair autosome chromosome number decrease in 18th pair autosome none of the above Reveal answer Fill a bubble to check yourself A Correct answer Explanation Down syndrome (DS or DNS), also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21.