Down Syndrome is caused by genetics. This means that the parents have passed down some form of a gene to their child. The child has a third copy of chromosome 21. It all revolves around this chromosome 21. Those without Down Syndrome do not have this chromosome 21.
The symptoms include growing slowly or there being a delay in the growth of someone with Down Syndrome, intellectual disabilities and certain facial features. Older mothers who have babies later in life like in their 40s stand a greater chance of having a child with Down Syndrome. Also, if the parent already has a child with Down Syndrome, there is a greater chance that they will have another child with Down Syndrome.