Down syndrome is due to something called "trisomy 21" - the affected individual has an extra copy of chromosome 21, which is the result of a problem with how the chromosomes pair up during meiotic cell division. Even though we know what the "genetic basis" is for Down syndrome, we don't know WHY an extra copy of chromosome 21 leads to the problems (which include heart & gut abnormalities, lack of nasal bone; cognitive problems).

In the region of chromosome 21 that is present in extra copy, there are many genes that now probably are over expressed (there is a "dosage" problem). Some of these genes encode regulatory proteins that then go on to regulate other genes (on other chromosomes) and thus a vast number of key genes probably are misregulated, leading to the "syndrome."

In theory, if we could identify which genes on #21 are the crucial players, it might be possible to devise a way to "shut down" or regulate expression of those genes in an affected individual early in development (in uterus). Since it is possible to do a quick genetic test via amniocentesis early in a pregnancy, identification of the trisomy is not the problem. The real challenge is "how to shut them down."

We could think about gene therapy or perhaps drugs that targeted the proteins, but each of these comes with problems. A "cure" is a long way off. Even in "simple" genetic diseases which are clearly linked to a defect in a single gene (not many genes, such as in the trisomy 21), we don't have a safe, effective way to correct the abnormal gene (think about sickle cell anemia, Huntington's disease - we know the exact genetic problem, but have no way to fix it yet).

There is a mouse strain that has been bred to serve as a model for Down's syndrome, and researchers are actively pursuing the question of the mechanism of the trisomy 21 defects.

Also, just for your information, most early embryonic and fetal demise likely due to chromosomal abnormalities (miscarriages) and about 50% of human ova (oocytes) have chromosomal abnormalities. Most of these chromosomal abnormalities result in very abnormal development early (first few days) and the embryos never implant or develop very far before miscarriage (spontaneous abortion). Many women never even know that they are pregnant before this happens. People with Down syndrome (trisomy 21) are some of the only people to survive beyond first weeks of infancy with a serious chromosomal abnormality.