Most people are probably not aware that there are three different types of Down syndrome. In fact, most people probably aren't exactly sure what Down syndrome is in the first place. To be brief, Down syndrome is a genetic condition which is caused by the presence of an extra chromosome in any or all cells of the human body. Specifically, it is caused by an added 21st chromosome. Typically a person is born with 46 chromosomes, 23 from the egg and 23 from the sperm. A person with Down syndrome has 47. The way this extra chromosome arrives and presents itself determines the type of Down syndrome a person has. It gets a little confusing but I will do my best to explain it clearly.
The type of Down syndrome L has is the most common, accounting for around 95% of babies with Down syndrome. It is called Trisomy 21, or Nondisjunction, and is decided before fertilization. It is set in motion during the division of the parent's sperm or egg cells. Typically a cell divides into two cells each containing 23 chromosomes. For T21 to occur, a cell divides into a pair of cells, one containing 22 chromosomes, and one containing 24. The cell containing 22 chromosomes cannot survive. However, the cell containing 24 chromosomes can survive. This cell containing the triplicate 21st chromosome happens to fertilize or be fertilized, resulting in a person with Down syndrome. When this type of Down syndrome occurs, each cell in the baby's body contains an extra copy of the 21st chromosome.
The second most common type of Down syndrome, making up 4-5%, is called Translocation. Translocation Down syndrome occurs when the 21st chromosome, or a piece of the 21st chromosome, breaks off and adheres itself to another chromosome (usually the 14th chromosome). The person with Down syndrome in this case has the typical pair of 21st chromosomes, but also has a 21st chromosome (or in rare cases, a partial 21st chromosome) attached to another chromosome. This usually happens spontaneously during fertilization, although around 25% of the time it happens because one of the parents is a carrier. Being a carrier means that in your genetic makeup you have the typical amount of chromosomes, but two of them are stuck together. This won't affect you, but can cause Down syndrome in your child. This also means you have a higher chance than the rest of the population to have a baby with Down syndrome. If a baby has translocation Down syndrome, the parents may choose to get tested to see if one of them is a carrier.
The third and rarest form of Down syndrome is Mosaicism, or mosaic Down syndrome. In mosaic Down syndrome, only some of the cells have an extra copy of the 21st chromosome. Like Trisomy 21, mosaic Down syndrome is caused by uneven cell division. There are two different ways that mosaic Down syndrome can occur, however, and each are slightly different than Trisomy 21. The first way happens when a cell divides unevenly in the earliest stages after fertilization. This means that every cell stemming from the improper division will contain the extra 21st chromosome. Some cells, however, did divide evenly, so all cells stemming from those will contain only two 21st chromosomes. The other way that mosaic Down syndrome occurs begins like Trisomy 21, but during cell division one cell corrects itself and all cells stemming from the corrected cell will not have the extra copy of the 21st chromosome.
I was recently made aware that some people have the idea that Down syndrome has something to do with the health or habits of the parents before and/or during pregnancy. Hopefully with education we can eliminate this notion from people's minds. You can neither prevent, nor cause Down syndrome on your own. It is decided genetically before (or in the extremely early stages after) fertilization, before implantation has even occurred.
Most of this information I learned from a book called Babies with Down Syndrome: A New Parents' Guide, which I highly recommend.