What is the Cause of Down Syndrome? What Makes People with Down Syndrome so Different? Down syndrome is a genetic difference caused by an extra chromosome. There can be many symptoms, and symptoms vary from person to person. Some people finish school and go on to college. Others never learn how to talk properly. Since chromosomes are what tell your cells what to do, when there is an extra, there can be conflicting instructions and/or the development in their mind and body is different. That's why there are also recognizable physical differences in a child or person with Down Syndrome.
Down Syndrome is not contagious. As a genetic difference, there is no way for it to "infect" another person or creature with Down Syndrome, simply because Down Syndrome is not a disease.
How does someone get this Syndrome? Extra chromosomes in some cells—This type is called Mosaic Trisomy 21. Part of the chromosome breaking off and reattaching to another chromosome—This type is called translocation trisomy. In about one third of individuals, the translocation is inherited from a parent. The estimated incidence of Down Syndrome is between 1 in 1,000 to 1 in 1,100 live births worldwide.
Below is a photo of my little brother with Down Syndrome.