So what causes Down Syndrome?

I realize not all my "31 for 21" posts have been about Down Syndrome. Hey, the rules of the game are that you can blog about ANYTHING, it doesn't have to be about DS. LOL

Anyway, I figured since this is Down Syndrome awareness month, I'd actually post something about DS, and what better thing to post about than a little explanation about what causes it? Please keep in mind this is a VERY simple explanation for those who are unfamiliar with DS.

Everybody is made of lots of tiny pieces, called chromosomes, and each chromosome carries the genes that make up who we are and what we look like, and control how our body develops. The average, genetically "normal" person is made up 46 chromosomes, which are matched up into pairs. So everyone is supposed to have a PAIR of Chromosome 1, PAIR of chromsome 2, etc, which comes out to 23 PAIRS of chromosomes.

But for people with Down Syndrome, at the moment of conception, as mom and dad's genetic material was exchanged, there was an error in duplication and the baby got an extra 21s chromosome. So, instead of 2, there are now 3. So, where the average person has 46 chromosomes, people with Down Syndrome have 47. Here's a picture of what the chromosomes look like. Notice the arrow that points to the extra 21st chromosome.


This is called Non-Disjunction Trisomy 21, and is the most common form of the syndrome, and every cell in the person's body carries the extra chromosome.

Sometimes that extra chromosome is sneaky, and at the moment of conception, instead of sitting next to it's matches, it attaches itself elsewhere, most commonly to the 14th pair. Sometimes it can stick ONTO one of the other 21st chromosomes. It doesn't just sit NEXT to them, no it sticks ONTO one of them. This is called Translocation Trisomy 21, and like non-disjunction, every cell in the body carries the extra pair. But chromosomes are touchy little things, and sometimes the extra piece attaches to different pairs, which are considered to be incompatible with life. For some reason those other pairs don't do well with extra material attached to them, and those babies don't survive.

There is one more type of Down Syndrome called Mosaic Trisomy 21. Where Non-Disjunction and Translocation happen at the moment of conception, Mosiac Down Syndrome (MDS) happens at the 2nd or 3rd cell division. That means that there will be some cells in the body that carry it, and some that do not. Some people who have MDS don't have the same features that others with DS do, and other people with MDS have stronger features. Ya just never know! People often ask me if Angela has MDS because of her mild facial features, but no...Angela has the garden variety Non-Disjunction Trisomy 21.

So what do the chromosomes DO? Well, within those chromosomes are smaller pieces called genes, which make up who you are, what you look like, your personality, etc. We know that the 21st chromosome affects organ development, because when there's an extra copy and Down Syndrome occurs, there are lots of heart defects, digestive malformations, etc. There are also distinctive physical characteristics as well. But every child is different, and where my child with DS has no heart defect, the next child might have one or several all at once.

Thanks to Down Syndrome, scientists know that Alzheimers is also controlled by the 21st chromosome. They know this because a very high number of people with DS develop Alzheimers. They also know that somehow cancer is connected to the 21st chromosome, because people with DS have a higher incidence of certain types of cancers, and for some reason they also have a higher cure rate. Now if only scientists could get it all sorted out, maybe they could find a cure.

So that, my friends, is my very short (lol) very simplified explanation of Down Syndrome.

*Karyotype picture from www.clinicaltools.com