One of the most intimidating things about being a mom to a daughter with Down Syndrome is being forced to study some chemical topics that are far beyond my current ability. I last studied chemistry back in college, some 30 years ago. Other moms have helped by studying, researching, and then translating what they know into simple terms. The best explanations from the best mom-teachers are here.
If you want to understand what that extra chromosome is doing in the cells of someone with Down syndrome, and what you can do about this problem, begin with these articles.