Down Syndrome


Intellectual Disability


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)]


In every cell in the human body there is a nucleus, where genetic material is stored in genes.  Genes carry the codes responsible for all of our inherited traits and are groups along rod-like structures called chromosomes.  Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent.  Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21 in all or some of their cells. This extra genetic material alters the course of development and causes the characteristics associated with Down syndrome. 

(National Down Syndrome Society, NDSS, 2019)


  • Trisomy 21 Down syndrome (DS)
  • Translocation Down syndrome (TDS)
  • Mosaic Down syndrome (MDS)

Each person with Down syndrome is a unique individual and characteristics associated with types of Down syndrome and the person’s intellectual disability will vary from mild to severe, with most individuals experiencing mild to moderate intellectual disabilities.    


Nearly 6,000 babies are born with Down syndrome in the United States each year. This means that 1 in every 700 babies is born with this condition. Although parents of any age may have a child with Down syndrome, due to higher birth rates in younger women, 80% are born to women under the age of 35. Its most common forms usually do not occur more than once in a family.

Down syndrome occurs in people of all races and economic levels, though older women have an increased chance of having a child with Down syndrome. A 35-year-old woman has about a one in 350 chance of conceiving a child with Down syndrome, and this chance increases gradually to 1 in 100 by age 40. At age 45, the incidence becomes approximately 1 in 30.

(Center for Parent Information and Resources, CPIR, 2017)


Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome.

The cause of the extra full or partial chromosome is still unknown. Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism. 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. The additional partial or full copy of the 21st chromosome which cases Down syndrome can originate from either the father or the mother. Approximately 5% of the cases have been traced to the father.

All 3 types of Down syndrome are genetic conditions (relating to the genes), but only 1% of all cases of Down syndrome have a hereditary component (passed from parent to child through genes). Heredity is not a factor in trisomy 21 (nondisjunction) and mosaicism. However, in 1/3 of cases of Down syndrome resulting from translocation there is a hereditary component – accounting for about 1% of all cases of Down syndrome.

Once a woman has given birth to a baby with Trisomy 21 (nondisjunction) or translocation, it is estimated that her chances of having another baby with Trisomy 21 is 1 in 100 up to age 40. 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.

Trisomy 21 (Nondisjuction)

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.


Mosaicism (or mosaic Down syndrome) 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 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.

(NDSS, 2019)


There is no method of prevention for the chromosomal condition associated with 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 unique and may possess the characteristics listed below to vary degrees. Some common characteristics include:

  • Poor muscle tone
  • Slanting eyes with folds of skin at the inner corners (epicanthal folds)
  • Hyperflexibility (excessive ability to extend the joints)
  • Short, broad hands with a single crease across the palm on one or both hands
  • Broad feet with short toes
  • Flat bridge of the nose
  • Short, low-set ears
  • Short neck and small head
  • Small oral cavity
  • Short, high-pitched cries in infancY 

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

(CPIR, 2017)


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 grow 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.

(CPIR, 2017)


Due to advances in medical technology, life expectancy for people with Down syndrome has increased dramatically.  With the discovery of antibiotics, and particularly advances in corrective heart surgeries, as many as 80% of adults with Down syndrome reach age 60, and many live longer.   As individuals with Down syndrome are living longer, they have increased interactions in schools, communities, and workplaces, thus increasing the need for widespread public education and acceptance. 

(NDSS, 2019)


Down syndrome is named after the English physician John Down, who characterized the condition.  The preferred language and spelling in the United States is “Down syndrome” not Down’s syndrome.  (NDSS, 2019).

“The National Down Syndrome Society advocates for education laws and policies that uphold the rights of individuals with Down syndrome to receive a free and appropriate public education in the least restrictive setting, promote principles and practices designed to improve educational outcomes for students with Down syndrome, protect the safety and dignity of students with Down syndrome and support the development of high quality postsecondary programs for students with intellectual disabilities.” (NDSS, 2019)

All people with Down syndrome experience cognitive delays, but the effect is usually mild to moderate. The CPIR presents ideas for teachers and education programs to address each student’s individual needs.

  • Learn as much as possible about Down syndrome.
  • Sometimes the appearance of Down syndrome can give the mistaken impression that the child cannot learn. Focus on the individual child and learn firsthand what needs and capabilities he or she has.
  • Realize that you can make a big difference in this student's life! Use the student's abilities and interests to involve and motivate. Give lots of opportunities for the student to be successful. Talk candidly with your student's parents. They're experts and can tell you a great deal about their daughter's or son's special needs and abilities.
  • Work with the student's parents and other school personnel to develop and implement a special education plan (IEP) that addresses the individual needs of the student. Share information on a regular basis with parents about how things are going for the student at home and in school.
  • If you're not part of the student's IEP team, ask for a copy of this important document. The student's educational goals will be listed there, as will the services and accommodations that he or she is supposed to receive, including in your class. 
  • Talk to specialists in your school (for example, special education), as necessary. They can help you identify methods that are effective for teaching a student with disabilities, ways to adapt the curriculum, and how to address the student's IEP goals in the classroom.
  • Emphasize concrete concepts over abstract ideas. Demonstrate what you want to see happen instead of giving only verbal instructions. When you share concrete information verbally, also show a photography. Give the student practical materials and experiences and the opportunity to touch and examine objects. 
  • Divide new tasks and large tasks into smaller steps. Demonstrate the steps. Have the student do the steps, one by one. Offer help when necessary.
  • Give the student immediate, concrete feedback. 

(CPIR, 2017)



International Mosaic Down Syndrome Association (IMDSA)
PO Box 321
Stow, MA 01775

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 (NDSC)
30 Mansell Court, Suite 108
Roswell, GA 30076
800.323.NDSC (6372)


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 (NDSS)
8 E 41st Street, 8th floor
New York, NY 10017


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 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.


Last modified October 2019.