Down syndrome is a polyploidy (three chromosome 21s)and is considered to be a somatic disorder. The other three are genetic because they are present in the form of a defective allele on achromosome.
gene |jēn|noun Biology (in informal use) a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring: proteins coded directly by genes.• (in technical use) a distinct sequence of nucleotides forming part of achromosome, the order of which determines the order of monomers in a polypeptide or nucleic acid molecule which a cell (or virus) maysynthesize.ORIGIN early 20th cent.: from German Gen, from Pangen, a supposed ultimate unit of heredity (from Greek pan- 'all' + genos 'race, kind, offspring' ).