"Down syndrome is a chromosomal disorder"
It is not a genetic syndrome. It is caused when an ovum separates unevenly, usually because it has been around for too long. There is a very rare genetic type, presumably originally caused when a Down syndrome person has a child and that child then has only half his or her cells with the extra chromosome, and then that child has a child, and so on, with the faulty reproductive cells getting more and more "watered down" through the generations.
At least one freeper became worried that Down syndrome ran in his family when a relative had a Down syndrome child. This is not something to make foolish errors about, as it can cause mental suffering and may even persuade someone unnecessarily not to have a child. There is a test for the genetic variety if one wants to make sure of that--as I said, very rare.
How common is Down syndrome?"Down syndrome is one of the most common genetic birth defects. It affects about 1 in 800 (about 3,400) babies a year in the United States (1). According to the National Down Syndrome Society, there are more than 400,000 individuals with Down syndrome in the United States"