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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.
Research shows that three types of chromosomal changes can lead to Down syndrome. 1  Complete trisomy 21. 2  In this case, an error during the formation of the egg or the sperm results in either one having an extra chromosome. 3  So after the egg and sperm unite, the resulting cells will also have three copies of chromosome 21. The complete extra copy of chromosome 21 is in all of the person's cellsâor a complete trisomy. Complete trisomy 21 is the cause of about 95% of Down syndrome cases. Mosaic trisomy 21.