system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do down sydrome kids all lookalike in regards to their facial proportions?
gpt
Because DS is caused by only one specific genetic mutation (an extra chromosome 21) it causes the same changes in the face in every affected person, to varying degrees. They have smaller mouths and bigger tongues, the bridge of the nose is flattened, the outer corner of the eyes turns up rather than down. These things together make what you are thinking of as the characteristic down syndrome face. 


Nobody yet knows why the extra chromosome does these things but groups of seemingly unrelated physical characteristics (like whorls in hair or wide spaced toes) are often specific to a particular genetic syndrome and used to diagnose it.