Down syndrome is usually caused by an error in cell division called "nondisjunction." Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down syndrome, which accounts for 95% of cases, is called Trisomy 21.
The two other types of Down syndrome are called mosaicism and translocation. Mosaicism occurs when nondisjunction of chromosome 21 takes place in one-but not all-of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. Mosaicism accounts for about 1% of all cases of Down syndrome. (http://www.uky.edu/DSAging/what.htm)
The Medical Genetics department at The Cleveland Clinic emailed me this photo of Luke's DNA today, via his final Karyotype study, showing the three copies of chromosome # 21. It was confirmed that Luke has the most common type of T21, "Nondisjunction". Pretty cool to be able to take a look at one of his actual cells!
As I was researching information on Down Syndrome today, I came across this website stating current clinical trials seeking individuals with DS. I'm including the link for future reference: clinicaltrials.gov.