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How does the presence of an extra chromosome 21 result in the characteristic craniofacial findings, developmental errors and other health issues?

I'd like to have an exact answer, because I can't seem to find a precise answer anywhere online, it seems all they mention is the fact that nondisjunction occurred during meiosis I. But I would like to know is why this extra chromosome creating so much complications?

It wouldn't make sense to talk about the position effect, so then I'd hypothesize that all the genes on chromosome 21 are over-expressed. How does this cause all the symptoms of Down's syndrome?

Thank you very much!

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closed as too broad by anongoodnurse, Bryan Krause, David, Remi.b, AliceD Oct 4 '17 at 21:51

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ wikipedia has 4-5 in depth references about the over expression of genes generating extra quantities of molecules and hormones, causing growth difference and symptoms similar to Alzheimer. en.wikipedia.org/wiki/Down_syndrome#Mechanism there's a lot of research about down's syndrome and genes on google scholar. $\endgroup$ – com.prehensible Sep 26 '17 at 13:30

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