Down Syndrome, also known as Trisomy 21, occurs when a child is born with partial or an extra copy of the 21st chromosome. This extra chromosome results in an altered course of development, and causes traits such as small stature, an upward slant to the eyes, low muscle tone, flat facial features, a single deep crease across the center of the palm, congenital heart problems, immune defects, mild to moderate intellectual disabilities, increased risk of dementia and several other features (WebMD, Understanding Down Syndrome). Down Syndrome is caused by an error in cell division called non-disjunction. Prior to, or during conception, a pair of 21st chromosomes fails to separate either in the egg or sperm. The embryo will end up having an extra, or part of an extra 21st chromosome (National Down Syndrome Society, What is Down Syndrome?).