Not sure why you're getting downvoted. It's a legit question for someone who does not know, as I did not before looking it up.
From the mayoclinic:
Most of the time, Down syndrome isn't inherited. It's caused by a mistake in cell division during early development of the fetus.
Translocation Down syndrome can be passed from parent to child. However, only about 3 to 4 percent of children with Down syndrome have translocation and only some of them inherited it from one of their parents.
Otherwise if down syndrome was spread by a single gene, it would have died out pretty naturally on its own, right?
(disclaimer) I admittedly don't know much about genetics.