Alternative Names
Trisomy 21; Down's Syndrome; 47,XX,+ 21; 47,XY,+21
Symptoms & Characteristics
Down syndrome (often also known as Trisomy 21) is associated with mental retardation, a characteristic facial appearance, and low muscle tone (called hypotonia) in infancy. People affected with this condition are at an increased risk for congenital heart defects, digestive problems (such as gastroesophageal reflux or celiac disease), hearing loss, and cancer of blood-forming tissue (leukemia). Additionally, some people affected with Down syndrome have reduced activity of the thyroid gland (called hypothyroidism).
Some evidence indicates that people with Down syndrome have an increased risk to develop Alzheimer's disease. Although Alzheimer's disease is usually a disorder of late adulthood, about 50% of people with Down syndrome have signs and symptoms of Alzheimer's disease by age 50.
Treatment
There is no cure for Down syndrome. However, early diagnosis, routine surveillance and treatment can help to manage some of the symptoms.
How Common Is It?
Down syndrome occurs in approximately 1 in 800 people worldwide.
Genetics & Inheritance
All of our genes are packaged together on chromosomes. People typically have 46 chromosomes in total, which are grouped into 23 pairs. The first 22 pairs are numbered largest to smallest (1-22) and are called autosomes. The last pair are called the sex chromosomes and determine gender, either XX (female) or XY (male). We typically inherit 23 chromosomes (one of each pair) from our mother (in an egg cell) and the other 23 from father (in a sperm cell).
Down syndrome is caused by the presence of an extra copy of chromosome 21 in every cell of a person's body.
In about 95% of cases, the extra copy is caused by a random "non-disjunction event" in either the egg or the sperm cell.
- Non-disjunction occurs when chromosomes in the developing egg or sperm fail to separate during cell division. The result is a sperm or egg cell with either an additional chromosome or one that lacks a chromosome. When this sperm or egg cell combines with one from the opposite sex, the resulting embryo will have cells with an extra chromosome (47) or cells that have a missing chromosome (45).
- The chance for a "non-disjunction event" to occur in an egg increases with age of the biological mother (maternal age). As a consequence, so does the risk for Down syndrome and other chromosome trisomies.
In 2-3% of cases, the extra copy is caused by a chromosome translocation. These cases are referred to as Translocation Down syndrome.
- This type of chromosome rearrangement can be inherited from a parent. An unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra or missing genetic material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk to have children with Down syndrome. People who carry this type of balanced translocation may also be at increased risk to have pregnancy losses, stillbirths, and children with other chromosome abnormalities.
- Parents and family members of a person with Translocation Down syndrome may want to consider chromosome analyses themselves. This testing should be discussed with a qualified health care professional.
In 1-2% of cases, the extra copy is caused by a random error during cell division early in fetal development (after conception has already occurred). As a result, only some (not all) of the body's cells have the usual two copies of chromosome 21, and other cells have three copies of chromosome 21. This is called Mosaic Down syndrome.
In rare cases, only part of chromosome 21 will be duplicated. This will lead to extra copies of some, but not all, of the genes on chromosome 21.
There are other genetic conditions that have symptoms that overlap with Down syndrome. A genetic consultation with a trained genetic professional is important for a complete evaluation, accurate diagnosis, as well as discussion of the benefits and limitations of chromosome testing and recurrence risk.
Genetic Testing
Down syndrome can be diagnosed by a chromosome analysis, which is available through an in person genetic consultation for people who are considered at risk. Use our find a genetic professional directory to locate a trained genetic professional in your area.
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