Causes of Down Syndrome

Down syndrome occurs in one of about eight hundred live births, and is caused by a chromosomal abnormality in the baby that grows more common as the mother ages. To give an idea of how pronounced this difference is, when a woman is in her twenties, the chances of having a Down syndrome baby is about one in one thousand; a woman over the age of forty has about a one in forty chance. But because the birth rate in older women is relatively low, approximately eighty percent of Down syndrome babies are born to mothers under the age of 35.

Every human is born with 23 chromosome pairs in each cell, for a total of 46 chromosomes. Down syndrome occurs when something goes wrong in the development of either the egg or sperm (usually the egg – 95% of Down syndrome cases come from the maternal genes), and chromosome 21 does not separate properly. Instead, the sex cell retains an extra chromosome 21. When the egg is fertilized, the cell contains three, not two, chromosome 21’s, for a total of 47 chromosomes. As the baby develops, this abnormality leads to the set of traits that collectively are described as Down syndrome.

In about four percent of Down syndrome babies, there are still the normal 46 chromosomes, but a piece of chromosome 21 has become detached and has reattached itself to a different chromosome. This is called translocation. There is no difference in the Down syndrome symptoms of babies in this variation. And in a very few Down syndrome babies, the syndrome is called “mosaic” because some of the cells have the extra chromosome 21 while others do not.

Unfortunately, we don’t know why the extra chromosome stays in the cell, or exactly how it causes Down syndrome. It is known that the extra chromosome causes the cells to generate an extra dose of certain proteins, but not which proteins, or how they’re involved in causing Down syndrome. It is also unknown when the extra proteins cause damage, though it’s almost certainly before birth.

There is no proven treatment for Down syndrome, as it is a genetic disease. Just as one cannot change his genes to have blonde hair, one cannot change the genes that cause Down syndrome. A girl with Down syndrome can have children, and has a fifty percent chance of passing the disorder to her children. Most boys with Down syndrome are infertile.