What are the signs and symptoms of Down syndrome?
A few of the common physical traits of Down syndrome are low muscle tone, small stature, an upward slant to the eyes, and a single transverse crease across the center of the palm – although each person with Down syndrome is a unique individual and may possess these characteristics to different degrees, or not at all.
Are there different types of Down syndrome?
There are three types of Down syndrome: trisomy 21 (nondisjunction) accounts for 95% of cases, translocation accounts for about 4% and mosaicism accounts for about 1%.
- Trisomy 21 (Nondisjunction) happens when an extra copy of chromosome 21 is present in all cells of the body. Chromosomes contain all of the genetic information that tells our body how to grow. Most people have 46 total chromosomes (23 pairs) in every cell in their body. One chromosome of each pair comes from the father, and one chromosome of each pair comes from the mother. An extra copy of chromosome 21 causes the differences we see in people with Down syndrome.
- Translocation Down syndrome happens when an extra copy of chromosome 21 is attached to another chromosome. This is then present in all cells of the body. 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 14. While the total number of chromosomes in the cells remains 46, the presence of any extra parts of chromosome 21 causes the characteristics of Down syndrome. Translocation Down syndrome is the only type of Down syndrome that can be passed down from a parent who does not have features of Down syndrome. (A consult with a genetic counselor or geneticist is important to investigate the risk of recurrence).
- Mosaic Down syndrome happens when an extra copy of chromosome 21 is present in some, but not all, of the body’s cells. About 1 to 2% of people with Down syndrome have mosaic Down syndrome. Mosaic Down syndrome is caused by a random event shortly after the egg and sperm join together. When cells are dividing, some cells receive an extra copy of chromosome 21. Others cells have two copies. Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome.