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From what is known about the causes of Down syndrome, environmental factors and parents' behavioral activity do not seem to play a role. The causes of Down syndrome are genetic problems related to chromosome 21. While Down syndrome is genetic in that it involves an extra chromosome 21, it is not usually hereditary in the conventional sense.
Like Patau syndrome, Down syndrome is caused by having an extra copy of a certain chromosome. Individuals with Down syndrome usually have an extra copy of chromosome 21. Thus, Down syndrome is commonly called trisomy 21. Perhaps not surprisingly, the cause of the trisomy in both Patau and Down syndromes is similar. A phenomenon called nondisjunction is responsible for most cases of trisomy 13 and 21.