Overview
Down syndrome, also known as trisomy 21, is a genetic disorder caused by a full or partial extra copy of chromosome 21. Chromosomes are long, tightly coiled strands of DNA. Each chromosome contains many genes and exists in pairs. Typically, human cells have 23 pairs of chromosomes or 46 in total, half of which are inherited from each biological parent. This additional chromosome alters the course of development in a baby’s brain and body, causing both intellectual and physical challenges to the baby.
People with Down Syndrome usually have a low range of intelligence quotient (IQ) and some common physical characteristics:
- Low muscle tone
- Hyper flexibility
- Flat face
- Upward slanted eyes
- Shorter height
- Smaller ears
- Smaller head
- Shorter neck
- Protruding tongue
- A single palmar crease
Prevalence
Down syndrome (DS) is the most common chromosomal abnormality. It happens to people of all races, ethnicities, geographic locations, and socio-economical levels. About 6,000 babies are born with the disorder each year in the United States,1 and 3,000-5,000 babies worldwide.2
Causes and Risk Factors
Researchers know DS is caused by the extra copy of chromosome 21 in all or some of the cells. But they do not know what causes the extra copy of chromosome 21 or how many different factors are in play.
There are, however, several factors that may increase the risk of having a baby with DS.
- Women who become pregnant over the age of 35 have a higher risk of conceiving a child with DS..
- A parent with genetic translocation for DS can pass the gene to the children.
- Parents who already have a child with DS have a greater risk of having another child with the disorder.
Types
There are three types of Down Syndrome:
- Trisomy 21
Trisomy 21 is the most common type of Down Syndrome, affecting about 95 % of people with the disorder. Trisomy refers to the condition where an extra copy of a chromosome is present in all or some of the cells in the human body. With Trisomy 21, each cell in the human body has three copies of chromosome 21 rather than the typical two copies.
- Translocation Down Syndrome3
About 3% of people with Down Syndrome have translocation Down Syndrome. This type of Down syndrome happens when a partial or full extra copy of chromosome 21 is attached or “translocated” to a different chromosome. Translocation Down Syndrome can be passed from a parent to a child.
- Mosaic Down Syndrome
Mosaic Down Syndrome accounts for 2% of people with Down Syndrome, the least common type. People with this type of Down Syndrome have three copies of chromosome 21 in some of their cells but the usual two copies in other cells. People with Mosaic Down Syndrome may have fewer characteristics of Down Syndrome.
Diagnosis
During Pregnancy
Two types of tests are available to detect DS during pregnancy – screening tests and diagnostic tests.
- Prenatal Screening Test
Prenatal screening tests do not provide a definite diagnosis, but these are safer for the mother and baby. A screening test can inform whether a pregnancy has an increased likelihood of DS. It cannot, however, determine if DS is absolutely present.
- Prenatal Diagnostic Test
Prenatal diagnostic tests, on the other hand, can give a definite diagnosis. But these tests carry greater risk for the mother and her developing baby.
After Birth
The diagnosis of DS after birth is usually based on the presence of physical traits of DS, as indicated above. Because similar features may also be present in babies without DS, a chromosome analysis called a karyotype is done to confirm the diagnosis.
Find out more about Down Syndrome Diagnosis
Treatment
DS is a lifelong condition and cannot be cured. However, early intervention can make a significant difference in improving quality of life. Because each person with DS is unique in their symptoms, the treatment will vary depending on what will best meet their needs.
Final Words
People with DS are like everyone else – each person having unique gifts, strengths, and capabilities. It is important that family members encourage them to pursue their dreams, utilize their gifts, and reach their full potential.
Find out more about Down Syndrome Treatments
Additional Resources
Down Syndrome Affiliates in Action
Down Syndrome Medical Interest Group
Global Down Syndrome Foundation
International Mosaic Down Syndrome Association
National Down Syndrome Society
National Down Syndrome Congress
Centers for Disease Control and Prevention
Linda Crnic Institutes for Down Syndrome
1 CDC. (2019, December 4). Data and Statistics on Down Syndrome | CDC. Centers for Disease Control and Prevention. https://www.cdc.gov/ncbddd/birthdefects/downsyndrome/data.html
2 CDC. (2019, December 4). Data and Statistics on Down Syndrome | CDC. Centers for Disease Control and Prevention. https://www.cdc.gov/ncbddd/birthdefects/downsyndrome/data.html
3 Down syndrome. (n.d.). Retrieved June 27, 2022, from https://www.marchofdimes.org/complications/down-syndrome.aspx