system
Given the following post, respond as if you're commenting on Reddit:
human
Why do people with Down Syndrome look the same/very very similar?

I feel somewhat bad for asking this question, obviously I know they are all unique but they do seem to share very similar facial characteristics.
gpt
People with Down syndrome have similar features because of genetic abnormalities. Because genes are essentially the “code” for how your body is built, a large change in the code (via an extra chromosome 21 in the case of those with Down syndrome) causes specific features to emerge. The features are similar from person to person because the code contained within the extra chromosome is similar and is expressed similarly from person to person. 

If you look at people who suffer from some other genetic diseases (Patau, Edwards, DiGeorge, Williams, etc.), those individuals also share similar features that are distinct from individuals with Down syndrome (although there is some overlap from one syndrome to another). These syndromes all feature a specific gene disruption that results in shared characteristics of those who have the disease. 

If you do some image searching, you can find lots of examples of the more common characteristics for all of these conditions.