What is the risk of a trisomy 21 parent passing on the condition to his/her offspring? Should adults with Down syndrome be dissuaded from reproducing

Depends on type. Risk to offspring depends on parents inheritence pattern.
It happends. Males with down syndrome are essentially sterile.A few rare mosaic male down cases might be capable but this the exception. Females have a 50% risk of passing ds to an offspring, but actual outcome is a bit >50% normal because some ds pregnancies will terminate spontaneously. A simple plan for long term birth prevention is an important feature in ds care. Ob + pcp+ family discussion is needed early.