About 95 percent of the time, Down syndrome is caused by trisomy 21 — the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell. Mosaic Down syndrome.
What genetic defect causes Down syndrome?
Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is trisomy. Down syndrome is also referred to as Trisomy 21.
What is the most common genetic alteration in Downs syndrome?
Most cases of Down syndrome result from trisomy 21 , which means each cell in the body has three copies of chromosome 21 instead of the usual two copies.
What is the mildest form of Down syndrome?
People with mosaic Down syndrome often, but not always, have fewer symptoms of Down syndrome because some cells are normal.
Can Down syndrome look normal?
People with Down syndrome all look the same. There are certain physical characteristics that can occur. People with Down syndrome can have all of them or none. A person with Down syndrome will always look more like his or her close family than someone else with the condition.
Do Down syndrome babies have normal heart rates?
Fetal heart rate of the trisomic fetuses was distributed around the median with that of all Downs syndrome fetuses within the normal range. In one fetus with trisomy 18, the heart rate exceeded the 90th centile, in another it fell under the 10th centile.