What is it?
Down syndrome is a genetic disorder. It typically leads to delays in physical growth, characteristic facial features, and low intelligence even as an adult. In the United States, 20% of people with Down syndrome are in a paid job of some kind. Their life expectancy is 50-60 years. The syndrome affects about 0.1% of babies born each year.
What causes it?
We normally have two copies of each of our 23 chromosomes: one from our father and one from our mother. Down syndrome happens if we end up with three copies of the chromosome 21. This can take place during cell division when a pair of chromosomes is supposed to be separated from one another. If the separation doesn’t succeed, there will be one sex cell with two copies of the chromosome and one sex cell with none. This can take place in any chromosome, but apart from having three copies of chromosome 21 and a couple of other exceptions, it leads to the child dying before being born.
Most often the sperm cell from the father is normal with one copy of each chromosome, while the egg cell from the mother is abnormal. The risk of Down syndrome increases with the mother’s age – in 20-year-olds it’s about 0.1%, while in 45-year-olds it’s about 3%.
What happens during it?
Practically everything that happens in our bodies relies on proteins, and genes are the instructions for proteins. In Down syndrome there are three copies of each gene located in chromosome 21 instead of the normal two, and that leads to an increased production of these proteins. For example, a certain gene on chromosome 21 is found to be highly active in both the heart and the brain, and it is thought to play a key role in both producing the heart problems common in people with Down syndrome, and the intellectual disability.
What happens during it?
Down syndrome can’t be cured. Focusing on the education and proper care of the affected has, however, been shown to increase the quality of life. People with Down syndrome are at a higher risk for many diseases, and regular screening for these is recommended.
With the easiness of genetic screening, most cases of Down syndrome in developed countries are now detected before birth. In Europe, 92% of fetuses with Down syndrome are aborted.
Sources: https://en.wikipedia.org/wiki/Down_syndrome, http://emedicine.medscape.com/article/943216-overview#a3