1–2% of the observed Down syndromes. There is evidence that mosaic Down syndrome may produce less developmental delay, on average, than full trisomy 21. The following diagram shows two possible mechanisms leading to mosaic trisomy 21. Duplication of a portion of chromosome 21 Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21 (46,XX,dup(21q)). If the duplicated region has genes that are responsible for Down syndrome physical and mental characteristics, such individuals will show those characteristics. This cause is very rare and no rate estimates are possible. See also Aneuploidy References Down syndrome