From my understanding, most cases of DS occur from a nondisjunction that occurs in either the sperm or ovum BEFORE fertilization. Mosaicism in Down Syndrome happens when the nondisjunction occurs AFTER fertilization, in one of the first cell divisions. Some studies seem to show that people with mosaic DS function at higher levels than those with the classic Trisomy, but other studies show no difference. For more information, go to www.ds-health.com/mosaic.htm
where you will find a very good article.