Is Downs Syndrome contagious?

Down Syndrome is not contagious. It is not a disease, and cannot be cured by medicine or surgery.

In Down syndrome, 95% of all cases are caused by an error called non-disjunction - one cell has two 21st chromosomes instead of one; so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21.

This extra 21st chromosome is either present with the 21st pair itself where the total chromosomal number is 47, or attached to the 13th, 14th, or the 15th chromosome pair where the total chromosomal number may be 46 itself. So the conditions are called pure trisomy or translocation respectively. A condition called mosaicism exists which is due to the fact that only certain cell lines have abnormal cells and the rest do not have, for e.g., the skeletal system may have cells with trisomy-21 and the blood may have normal cells with 46 chromosomes without trisomy.

The cause of the nondisjunction error isn't known, but there is definitely connection with maternal age. Research is currently aimed at trying to determine the cause and timing of the nondisjunction event.