Down syndrome is the most common chromosome condition. It occurs in about 1 out of every 800 liveborn babies. Down syndrome occurs in all races and in all populations. People with Down syndrome have mental retardation, a similar appearance, and a higher chance for medical problems.
The majority of individuals with Down syndrome have mental retardation in the mild to moderate range. This means that these individuals can and will learn new skills although at a slower rate than someone without mental retardation. Children with Down syndrome are able to learn to walk, talk, and possibly read and write. Adults with Down syndrome may be able to live semi-independently.
There is a great deal of variability in how individuals with Down syndrome are affected physically. Individuals with Down syndrome have many common physical features that result in a distinct physical appearance. These features may include skin folds in the inside corners of the eyes, upslanting eyes, a somewhat flatter appearing face, a smaller mouth, and a protruding tongue. Health concerns are also more frequent. The medical problems found more often with Down syndrome include congenital heart defects, digestive problems, respiratory and ear infections, decreased functioning of the thyroid gland, lower muscle tone, and vision problems. Most people do not have all the physical problems possible in the condition; instead, they may have just a few health concerns or no major medical problems at all. In the absence of a major health concern, individuals with Down syndrome can live into their fifties and sixties.
Down syndrome is a genetic condition caused by an extra chromosome 21. The extra copy of chromosome 21 causes the characteristic features seen in individuals with this condition.
Chromosomes are the packages of DNA found in almost every cell of the body. Chromosomes carry genes which are the instructions for how our bodies grow and develop. Healthy individuals who do not have Down syndrome have a total of 46 chromosomes in each cell. The usual set of chromosomes includes 23 paired chromosomes (one copy from each parent) and the chromosomes are numbered by size. Sperm and egg cells are the exception to the rule and contain only half the usual number of chromosomes, for a total of 23 chromosomes (one copy of each).
Sometimes, errors in the cell division process can cause a sperm or egg cell to contain an extra chromosome; 24 chromosomes, rather than the usual 23. This error always occurs by chance and there is nothing a parent can do to cause or prevent this from happening. As a woman's age increases, she is more likely to have a baby with a chromosome abnormality, but these types of birth defects can happen at any age.
Types of Down syndrome
The features of Down syndrome are always due to the influence of the extra genetic material from chromosome 21. However, there are three different types of Down syndrome that can only be determined by the chromosome analysis.
Trisomy 21: The majority (about 95%) of individuals with Down syndrome have three separate copies of chromosome 21. This type of Down syndrome is called trisomy 21. It occurs randomly and does NOT run in families.
Mosaic Down syndrome: Another type of Down syndrome is called mosaic Down syndrome. In this situation, an individual has two different cell types- some containing 46 chromosomes, others containing 47. The cells with 47 chromosomes have an extra 21 chromosome which can cause varying degrees of Down syndrome depending on how many cells in the person's body have the extra chromosome 21. Mosaic Down syndrome is also not inherited and occurs by chance alone. It is a rare occurrence and is seen in approximately 1-2% of individuals with features of Down syndrome.
Translocation Down syndrome: About 3-4% of individuals with Down syndrome have the usual number of chromosomes in their cells but have an extra piece of chromosome 21 attached to, or translocated to, another chromosome. In some cases the chromosome translocation that causes Down syndrome is inherited and other family members could also be at risk to have children with the condition.
Although most cases of Down syndrome are NOT inherited, anyone with a family history of Down syndrome should contact a genetic counselor to determine their individual risk to have a child with Down syndrome.
Down syndrome is often suspected shortly after birth due to the characteristic facial features. To confirm a suspected diagnosis, a blood sample is obtained and the baby's chromosomes are studied. This testing is extremely accurate and can provide a definitive answer as to whether or not a baby has Down syndrome or other chromosome condition. If an individual has Down syndrome, the result from the chromosome analysis is also useful in determining the chance of having another child with Down syndrome.
Down syndrome can be detected through diagnostic tests, such as chorionic villus sampling (CVS) and amniocentesis. These diagnostic tests look directly at the fetal chromosomes, so they provide a definitive answer as to whether or not a fetus has Down syndrome. CVS is performed during the first trimester of pregnancy between 10-13 weeks. Amniocentesis is performed in the second trimester routinely between 15-20 weeks. Results of testing usually take about 2 weeks.
An alternative to diagnostic testing are prenatal screening tests. Screening tests provide a pregnancy-specific risk estimate for Down syndrome but do not provide a definitive answer. If the risk is increased, the option of diagnostic testing is offered in order to obtain a definitive answer.
At this time, there is no known cure for the condition and no known way to remove the extra genetic material from each cell. Physical complications can be treated medically or surgically when necessary. Special education programs are available even from infancy to determine areas of strengths and weaknesses in the child and maximize the child’s full potential.
Medline Plus: Down syndrome - General information about Down syndrome provided by the National Library of Health and the National Institutes of Health. Extensive links to related online resources.
National Down Syndrome Society - Provides information and support for individuals with Down syndrome and their families.
National Mosaic Down Syndrome Association - Parent support resources specifically about mosaic Down syndrome.