Sometimes, sometimes not. Always with nondysjunction. Partial trisomy does not affect every chromosome 21. Translocation does not result always result in trisomy but results in Downs, the extra #21 chromosome can attach elsewhere. Translocation is heritable.
It's not a point mutation if that's what you were arguing about.
Now you're on my side.
I'm not on anybody's side.
I provide information. Not everyone is at the same knowledge level on every subject and I find discussions go better when the participants have enough information to be on a par with one another.
In addition, because of the pervasive poor quality of teaching the basic sciences, many people think they know a good deal that is actually incorrect. It is not their fault if they trusted their teachers and it should not be used against them to score debate type points.