Down Syndrome


Intellectual Disability—Down Syndrome



Our nation’s special education law, the Individuals with Disabilities Education Act (IDEA) defines intellectual disability as...

“...significantly subaverage general intellectual functioning existing concurrently with deficits in adaptive behavior. And manifested during the developmental period that adversely affects a child's educational performance.”

[34 Code of Federal Regulations §300.8(c)(6)]

Other Definition

Down syndrome is the most common and readily identifiable chromosomal condition associated with intellectual disabilities. It is caused by a chromosomal abnormality: for some unknown reason, an accident in cell development results in 47 instead of 46 chromosomes. This extra chromosome changes the orderly development of the body and brain. In most cases, the diagnosis of Down syndrome is made according to results from a chromosome test administered shortly after birth. (NICHCY, 2010)


Nearly 5,000 babies are born with Down syndrome in the United States each year. This means that 1 in every 733 babies is born with this condition. Although parents of any age may have a child with Down syndrome, 80% are born to women under the age of 35. Down syndrome is not a disease, nor is it contagious. Its most common forms usually do not occur more than once in a family. (NICHCY, 2010)


Down syndrome is usually caused by an error in cell division called nondisjunction. Nondisjunction results in an embryo with three copies for chromosome 21 instead of two. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down syndrome, which accounts for 95% of cases is called Trisomy 21.

The other two types of Down syndrome are call mosaicism and translocation. Mosaicism occurs when nondisjunction of chromosome 21 takes place in one-but not all-of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47. Mosaicism accounts for about 1% of all cases of Down syndrome. Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess. 

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 chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.

The cause of nondisjunction is currently unknown, but research has shown that it increases in frequency as a woman ages. However, due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age. There is no definitive scientific research that indicates that Down syndrome is caused by environmental factors or the parents' activities before or during pregnancy.

Once a woman has given birth to a baby with Trisomy 21, it is estimated that her chances of having another baby with Trisomy 21 is 1% greater than her chances by age along. The age of the mother does not seem to be linked to the risk of translocation. Most cases are sporadic - that is, chance events. However, in about one third of cases, one parent is a carrier of a translocated chromosome. The risk of recurrence of translocation is about 3% if the father is the carrier and 10-15% if the mother is the carrier. Genetic counseling can determine the origin of translocation. (National Down Syndrome Society, 2012)


There is not method of prevention for the chromosomal accident that causes Down syndrome.


There are over 50 clinical signs of Down syndrome, but it is rare to find all or even most of them in one person. Every child with Down syndrome is different. Some common characteristics include:

Individuals with Down syndrome are usually smaller than their nondisabled peers, and their physical as well as intellectual development is slower. (NICHCY, 2010)


Besides having distinct physical appearance, children with Down syndrome frequently have specific health-related problems. A lowered resistance to infection makes these children more prone to respiratory problems. Visual problems such as crossed eyes and far- or near-sightedness are common in individuals with Down syndrome, as are mild to moderate hearing loss and speech difficulty. Approximately one third of babies born with Down syndrome have heart defects, most of which are now successfully correctable. Some individuals are born with gastrointestinal tract problems that can be surgically corrected. 

Some people with Down syndrome may also have a condition known as Alantoaxial Instability, a misalignment of the top two vertebrae of the neck. This condition makes these individuals more prone to injury if they participate in activities that overextend or flex the neck. Parents are urged to have their child examined by a physician to determine whether or not their child should be restricted from sports and activities which place stress on the neck. Although this misalignment is a potentially serious condition, proper diagnosis can help prevent serious injury.

Children with Down syndrome may have a tendency to become obese as they grown older. Besides having negative social implications, this weight gain threatens these individuals' health and longevity. A supervised diet and exercise program may help reduce this problem. (NICHCY, 2010)


Life expectancy for people with Down syndrome has increased dramatically in recent decades - from 25 in 1983 to 60 today. People with Down syndrome attend school, work, participate in decisions that affect them, and contribute to society in many ways. Quality educational programs, a stimulating home environment, good health care, and positive support from family, friends, and the community enable people with Down syndrome to develop their full potential and lead fulfilling lives. (National Down Syndrome Society, 2012)


All people with Down syndrome experience cognitive delays, but the effect is usually mild to moderate. Therefore, education programs will need to address each child's individual needs.
Tips for Teachers:


International Mosaic Down Syndrome Association
PO Box 1052
Franklin, TX 77856

The International Mosaic Down Syndrome Association is designed to assist any family or individual whose life has been affected by Mosaic Down Syndrome, assist in research, and provide support without regard to race, sex or religion. IMDSA strives to increase awareness in the medical, educational, and public communities throughout the world. The IMDSA website provides general information about DS and MDS specifically, as well as information about DS research and resources for families and educators.

National Down Syndrome Congress
1370 Center Drive, Suite 102
Atlanta, GA 30338

The mission of the National Down Syndrome Congress is to provide information, advocacy and support concerning all aspects of life for individuals with Down syndrome.  It is the purpose of the NDSC to create a national climate in which all people will recognize and embrace the value and dignity of people with Down syndrome. The NDSC website provides general information about DS, resources for parents and news about legislation concerning DS.

National Down Syndrome Society
666 Broadway
New York, NY 10012

The mission of the National Down Syndrome Society is to benefit people with Down syndrome and their families through national leadership in education, research and advocacy. A resource for families, professionals, affiliates and others, the NDSS Web site, receives 2.5 million page views each year and is one of the largest single sources of dependable information about Down syndrome available to the public. Updated daily, this site is a valuable tool for public education and information sharing.  The website also contains a page concerning Down syndrome and the arts which includes information about music and music therapy.