system
Given the following post, respond as if you're commenting on Reddit:
human
ELIF why do people with down syndrome look so similar?
gpt
Well, every person out there has a unique set of "blueprints" for how they are put together.  We call these instructions "DNA".

For people with down's syndrome, they all have an "extra" section of DNA that isn't supposed to be there.  Imagine you're building a house, and there's an extra drawing that shows another roof.  If you build the house from these blueprints, it'd have two roofs and look a little different than a normal house.  

Not only that, but if your neighbor made the same mistake and also had an extra drawing that shows another roof, then his house would have an extra roof too.  It'd look a lot like your house.  Then you would ask why everyone's house with two roofs look similar.  It's because the plans for all the houses all had the same type of mistake.

In people with Down's Syndrome, they all have the same bit of "Extra" DNA.