If you think of it like down syndrome, the extra chromosome is linked onto the 21 chromosme pair making it have 3 chromotids instead of 2. If there is one already being passed on, the chances of hitting all 4 together is extremely unlikely and the 4th copy would like deteriate. I studied downsyndrom scans, and even had to do "mock diagnoses" in my coursework at the univeristy based on this stuff, and the inheritance patterns of people with down syndrome. . . The difference is, downsyndrom is a random occurance, and not a gene that is continually being passed on. It's a disorder, that becomes heritable.