Thursday, October 22, 2009
October is National Down Syndrome Awareness Month. Our beautiful Lillie, born September 26, 2009, has Down Syndrome. In honor of her, I thought I'd tell you all a little something about Down Syndrome. You can learn more by visiting www.ndss.org.
Down syndrome occurs when an individual has three, rather than two, copies of the 21st chromosome. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome.
There are 3 types of Down syndrome. We do not yet know which type Lillie has. The nurse called and confirmed a positive diagnosis, but Lillie's doctor (who can actually understand the report) is out of the office for a couple of weeks, so we're waiting to learn more.
All Down syndrome is also called Trisomy 21. Here are the 3 types of Trisomy 21.
1. Nondisjunction is the most common type of Down Syndrome (95% of cases). It is 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.
2. 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.
3. Translocation accounts for about 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.
The only way to know which type of Down syndrome Lillie has is through a chromosome study (blood test). That's been done, and we're waiting on the Doctor to let us know what the results are.