Talk to Endocrinologist on Downs Syndrome

Dear doctors, I want to get married to a girl whose sister is afflicted with Downs syndrome and my parents are not allowing me to do so just because of the said reason otherwise she is all okay, and even more intelligent than me. My parents think that its genetic and could be passed down to my generation.

Could any of you suggest what to do? What are the myths? Will that affect our life? Should I marry her? Can pre-marriage counseling of us, two will be of help prior to conceiving a child?

Awaiting your responses.

Regards,