Why would anyone think that all the races of this world came from a single person? well, the chemosynthetic theory of origin of life and the evolution theory actually trolled that line. Chemosynthetic theory believes the first cell arose through mixture of chemical substances that were constituents of primitive atmosphere and that other cells arose from it. The evolutionists believes that organisms arose from a common ancestor but developed through different adaptive radiations.
Downs syndrome is caused by any extra genetic material relative to chromosome 21, and is usually caused by a chromosomal duplication event resulting in their being 3 copies of that chromosome rather then the inversion event you describe here.
Probably there are different genetic mechanisms behind down syndromes. I will read up :)