Ahhh ok thanks for that, i guess this extra chromosome adds the feature of smaller eyes and indented face structure whereas the other 'normal' chromosomes' function as normal in terms of height/hair colour etc.
Well it's not that they function as normal per se, there are variations within Down Syndrome people according to their other genetic influences. It's just like some with Down Syndrome are high functioning and others are severely developmentally delayed. Some show all the physical characteristics associated with Down Syndrome and some only show a few. It's a lottery.
If you got a bunch of us together in a room, discounting race, if you made our eyes smaller, gave us wide set noses, inverted faces, small ears etc we'd all look much more similar than we do now. I don't think it's so much as they look the same, but that because they're drastically different they just all look similar.
But yeah you've got me with that, and truthful answer is that for everything we know about Down Syndrome there is a bucket load more we don't.