Down's happens when the egg meets the sperm. It's not something that the mother or father carries in thier genetics.
Stuff like cystic fibrosis, sma, sickle cell, huntington's, fragile x, and a whole long list of other things are in the genetics of the parents. Some require a copy from the mother and father and some just requires one copy. Many people are carriers of genetic conditions, but you have to partner with someone that carries the same gene.
Now, many will use 23 and me and other chromosome testing companies to find out if they are a carrier and their partner is a carrier. If only one is, on a recessive gene, then they'll not do anything about it. 99% of couples do not terminate a pregnancy for a carrier child of a recessive disorder. Now dominate disorders, like huntington and fragile x, that's different.