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Northwest Kansas Down Syndrome Society The difference between ordinary and extraordinary is just that little extra Photo Credit Christine Roth Photography What is Down Syndrome. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21. The additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. A few of the common physical traits of Down syndrome are low muscle tone small stature an upward almond slant to the eyes and a single deep crease across the center of the palm.