Which condition is associated with an extra copy of chromosome #21?

Question

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An extra copy of chromosome #21 is characteristic of Down syndrome. Sickle-cell anemia is caused by a mutation in a specific gene on chromosome #11, ALS has been linked to mutations on multiple genes including C9orf72 and SOD1, epilepsy has been associated with mutations in various genes, and schizophrenia is believed to be caused by a combination of genetic and environmental factors.

Which Condition Is Associated with an Extra Copy of Chromosome