It is also important to note mosaic trisomy 21 is reported to be a vast minority in the literature. Certainly there seems to be a correlation between % trisomic cells and clinical phenotype / degree of mental retardation. However, perhaps even more interestingly is the huge variation in associated phenotypes complete trisomies. The number of associated phenotypes is huge, and two individuals with Down syndrome likely only share a small subset.

More importantly, this idea of genetic heterogeneity should be expanded to all diseases. Not just in the way of copy number variants, but also causative alleles. No two individuals have exactly the same course of any genetic disease. Furthermore, there are cell to cell differences throughout the body that differ in mutation content and copy number variation. You might have a cell in your thumb deleted for a colon cancer tumor suppressor, but if that gene doesn't perform that function in your thumb it doesn't matter.