One in a Million...

If I was a betting kind of person, I would think now is the time to start playing the lottery...

Yesterday we had an appointment with the geneticist and we finally found out what type of Down syndrome Sweet Pea has.  And, she of course has one of the rarest types.  

A little info on the different types of Down syndrome.  Hey, I was surprised to find this out, so I'm sure most of you would be too =)

First, Down syndrome is caused by a triplicate copy of the 21st chromosome.  It is also known as trisomy 21.

The most common form of Down syndrome is nondisjunction.  This form accounts for about 95% of all cases.  Nondisjunction is formed when one pair of chromosomes fails to separate evenly during meiosis (either the sperm or egg cell).  This causes the cell to have 24 chromosomes and when combined (at fertilization) with a *normal* cell (23 chromosomes), it results in a cell with 47 chromosomes instead of the normal 46.  Because this happens at fertilization, all the cells in the body will have 47 chromosomes.

The another form of Down syndrome is Mosaic Down syndrome.  Mosaicism accounts for about 2% of all cases of Down syndrome This is similar to nondisjunction, except the uneven separation occurs in one of the dividing cells of the fertilized egg.  As a result, the extra chromosome is not present in all cells, but rather in only a certain percentage of cells.  

The third for of Down syndrome is called translocation.  Translocation accounts for 3% of all Down syndrome cases.  Unlike the previous forms, the chromosome count on karyotypes is usually 46 chromosomes.  Why this is still a form of Down syndrome is because there is still a third copy of the 21st chromosome.  The chromosome is just attached (or translocated) to another chromosome.  The most common translocation is to the 14th chromosome, although it can also be found on chromosomes 13,15,21 or 22.  This is the only form of Down syndrome that can be inherited (about 25% of translocation Down syndrome is inherited).  In inherited cases, the parent with the extra copy is called a balanced carrier- meaning they have one chromosome fused together and on a karyotype will show 45 chromosomes instead of 46.  Because there is no extra genetic material, the parent will not have Down syndrome.

Confused yet?  Good =)

So, Sweet Pea has translocation Down Syndrome and her extra 21st chromosome is parked squarely on top of one of the other 21st chromosomes.  The rarest of the rare.  But what does this mean?  Well, nothing really beyond the fact that she has Down syndrome.  And the fact that she is 100% cute.

Practicing her pirate face...  Argh, matey!

However, if she had been our first child, it would have made a huge difference for us and any future kids.  If one of us had been a carrier of a (21;21) translocation, 100% of all of our other kids would have also had Down syndrome. No if, buts, hows or whys, that is just how the genetic deal works. 

According to the Dr yesterday, it is very unlikely that we are carriers. The fact that we have three other *normal* kids makes it almost 100% that this was just one of those things =)