system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the extra copy of Pair 21 cause down syndrome

wouldnt the extra chromosome help by providing extra dominant genes?
gpt
Aww shit son, buckle your gene belt.

So an extra chromosome equals an extra chunk of genetic material that gets expressed.

So that genetic material is getting processed and coding for a bunch of proteins that are already being produced by the other two copies.

And chromosome 21 doesn't have the inactivation ability like the X chromosome, so these proteins just keep getting pumped out, which has an effect on the developing child.

Tl;dr - extra chromosome makes too many proteins. Too many proteins make abnormal baby.