What is Down Syndrome?

(Brunton at only 4 hours old, pre-NICU)

So what exactly is Down Syndrome and what causes it? I had always thought that there was only one type of DS, but I was wrong yet again. There are actually three different forms of DS, Trisomy 21, Mosaicism, and Translocation.The most common is trisomy 21. Trisomy 21 accounts for about 95% of cases of Down Syndrome. This type of Down Syndrome occurs because of an error in cell division. What happens is that a pair of the 21st chromosomes in either the egg or the sperm do not separate properly. This extra chromosome is then found in every cell in the body,thereby causing the characteristics of Down Syndrome. Mosaicism occurs in about 1-2% of all cases of Down Syndrome. In this type of Down Syndrome the error in separation of the 21st chromosome occurs in one of the first few cell divisions AFTER fertilization. This causes the fetus to have some cells with 46 chromosomes and some with 47. The cells with 47 chromosomes have one extra 21st chromosome. Because not all of the cells contain the extra chromosome 21 the range of physical problems varies depending on the ratio of cells with 46 chromosomes to those with 47 chromosomes. Translocation occurs in approximately 3-4% of babies born with Down Syndrome. In this type of Down Syndrome a part of chromosome number 21 breaks off and attaches itself to another chromosome. Often this is chromosome number 14. This causes all cells in the body to have the extra piece of the 21st chromosome.
Brunton is believed to have Trisomy 21. But to know for sure we would have to do further genetic testing.