Down Syndrome

Down syndrome is a genetic * condition that occurs when a person has three copies of chromosome 21, rather than the usual two. People with Down syndrome usually have a characteristic physical appearance, significantly lowered intellectual abilities, and sometimes a number of physical problems, such as heart defects.

Anita: A Look at Someone with Down Syndrome

Jason's sister Anita is a happy, active, five-year-old girl. She also is a girl with Down syndrome, and Jason feels more protective of his sister than most big brothers are of their siblings. When they are out at the mall, everyone remarks about how adorable Anita is, but they also stare at her. Anita has many of the characteristic features of people with Down syndrome, and some of them are quite noticeable. These features may include the following:

These characteristics and their severity vary among affected people, and some people with Down syndrome have fewer of these features than others. Anita is somewhere in the middle of the range. She is also moderately learning disabled.

Jason did some research on Down syndrome for a school report and now worries about Anitás health. Many people with Down syndrome have health problems such as heart defects and increased susceptibility * to infection, respiratory problems, and digestive problems. Childhood leukemia occurs slightly more frequently in children with Down syndrome than in other children, and adults with the syndrome are at increased risk for Alzheimer's disease * .

What does life hold for Anita and her family? In 1910 most children with Down syndrome did not live past the age of nine years, often because of infections. When antibiotics were developed during the 1940s, the average child with Down syndrome survived to age 19 or 20 years. With recent advances in clinical treatments, as many as 80 percent of adults with Down syndrome live to 60 years and even longer.

Chart showing the 22 chromosome pairs, in this case there are three chromosomes instead of two in pair 21, an abnormality producing Down syndrome (trisomy 21).

Chart showing the 22 chromosome pairs, in this case there are three chromosomes instead of two in pair 21, an abnormality producing Down syndrome (trisomy 21).
Alila Medical Media/

Anita's family learned all they could about Down syndrome as soon as she was born. They knew Anita would learn to sit, walk, and talk somewhat later than her peers. Anita's family is providing her with a stimulating home environment, good medical care, and good educational programs. They are teaching her to be a happy and productive member of the community. Whether she will be able to go to school or to work or live independently depends on the level of her mental development. However, Anita's family knows that the Americans with Disabilities Act of 1990 helps to protect Anita's right to live her life free of unnecessary limitations or discrimination as a result of her disabilities.

What Is Down Syndrome?

All people have chromosomes in their cells. Chromosomes provide the genetic information needed for the cells of the body to work properly. Normally, a person's cells contain 23 pairs of chromosomes, for a total of 46 chromosomes. The exceptions are eggs and sperm cells, which have only one set of 23 chromosomes. People who have Down syndrome, however, have an extra copy of chromosome 21, or their chromosome 21 has extra parts.

No one knows what causes the chromosomal abnormality that results in Down syndrome. Any woman can have a baby with Down syndrome. It is not associated with a person's culture, race, geographic location, or relative wealth.

A mother's age, however, does seem to be correlated * with her risk of having a child with Down syndrome. Women age 36 years or older have a greater chance of giving birth to a baby with Down syndrome than women under 36 years of age, and the risk increases with every passing year.

Down syndrome can occur in three ways: nondisjunction, translocation, and mosaicism.


About 95 percent of people with Down syndrome have three copies of chromosome 21, a condition called trisomy 21 (tri means “three”). This condition occurs as a result of an error in the way that cells divide. This error is known as nondisjunction.

* and continue to make new cells with the extra copy of chromosome 21—for a total of 47 chromosomes rather than the normal 46—throughout its entire body.

1866: English physician, John Langdon Haydon Down (1828–1896), publishes the first description in the medical literature of a person with Down syndrome.

1959: Jérôme Lejeune (1926–1994), a French physician, finds the extra chromosome 21.

2000: Scientists unravel the genetic code of chromosome 21. Chromosome 21 is the smallest of all human chromosomes, and scientists believe it has 200 to 400 genes, each of which has a specific job to do. Some of these genes produce proteins, one of which regulates the autoimmune system, whereas other genes produce enzymes, one of which affects amino acids, which are the building blocks of proteins. When a cell has an extra chromosome, and therefore extra genes, the genes can make too much of their protein; the extra proteins can cause an abnormality.

2006: Researchers find that a gene on chromosome 21 seems to be responsible for learning disabilities among people who have Down syndrome. These scientists conducted their study on mice instead of humans, but they believe their results translate to humans. They found that mice with three copies of the gene made too much of a certain protein. The extra protein caused certain nerve cells to be especially large, and this, in turn, caused learning disabilities. Researchers later tried to figure out if they could find a way to “turn down” the gene so it makes less of its protein. Success means eventually using this knowledge to help treat people with the syndrome. In the meantime, scientists elsewhere are carefully studying other genes on chromosome 21 to learn the particular function of each, and possibly to find ways to treat or prevent Down syndrome.


Some patients with Down syndrome have the normal number of 46 chromosomes in their cells. This infrequent condition occurs because they still have some extra parts of chromosome 21 in their cells. The extra chromosome 21 results when a piece of it breaks off while a cell is making a copy of itself, and that extra piece attaches to another chromosome. This abnormal process is called translocation. In this case, the incorrectly attached genetic material from chromosome 21 causes the features of Down syndrome.


How Common Is Down Syndrome and Is It Contagious?

Down syndrome is the most common chromosomal abnormality in humans. It affects 1 in 700 babies in the United States. Researchers estimate the chance of having a baby with Down syndrome to be as follows:

Translocation accounts for only 3 to 4 percent of all cases of Down syndrome. Mosaicism is very rare, occurring in only 1 to 2 percent of people with Down syndrome.

Down syndrome is not contagious.

How Do Doctors Diagnose Down Syndrome?

Down syndrome is usually diagnosed either during pregnancy through routine prenatal testing or at birth, as newborns exhibit the distinctive physical features of the condition.

Medical professionals have several ways to test for Down syndrome, but this does not mean all pregnant women decide to undergo testing. Some women prefer not to know; many women do want to know. The reasons for getting tested may include early preparation for the child's medical care, making sure that funding or insurance to pay for the child's special needs is available, or, for women who decide for any number of reasons not to have a child with Down syndrome, ending the pregnancy.

Doctors commonly use the triple screen test or the alpha-fetoprotein * test to predict whether a woman is carrying a baby with Down syndrome. The tests are called screening tests because they do not give a definitive result. Instead, they measure the amounts of certain substances in the mother's blood that can indicate a problem. If one of these tests is positive, it does not necessarily mean the fetus (the developing baby) has Down syndrome, but it does indicate the woman should undergo more testing. Sometimes the test results are false negative, meaning the test did not indicate Down syndrome although the fetus has it. Low levels of alpha-fetoprotein in the mother's blood are correlated with Down syndrome in the fetus, but the test detects only about 35 percent of cases. The triple test, which measures levels of three substances, is correct about 60 percent of the time.

Another type of screening is called nuchal (NOO-kul) translucency screening. During this test, medical professionals perform an ultrasound, which uses sound waves, to examine the skin at the back of the fetus's neck. They look at the size of the clear space in the neck tissue. Babies with Down syndrome have extra fluid in this part of the neck, which makes the clear space look larger. This test is able to detect about 80 percent of Down syndrome cases.

Medical professionals recommend several different diagnostic tests for pregnant women older than 35 years, and for women with positive results on screening tests. A common diagnostic test is amniocentesis * . During this test, doctors examine chromosomes from the fetus's cells. This diagnostic test gives a definitive answer, which means the results are correct 98 to 99 percent of the time. During this test, medical professionals extract fetal cell samples that are taken from the fluid surrounding the fetus or from the umbilical cord * . On rare occasions, these procedures cause the mother to have a miscarriage (lose the baby before birth). Women who plan to have diagnostic tests performed should receive information and have emotional support available to help them understand the procedures and cope with the possibility of miscarriage, and also with the possibility of results indicating Down syndrome.


There is no treatment for Down syndrome.

Can Down Syndrome Be Prevented?

Down syndrome cannot be prevented.

See also Birth Defects: Overview • Genetic Diseases: Overview • Learning Disabilities: Overview


Books and Articles

Daugherty, Paul. An Uncomplicated Life: A Father's Memoir of His Exceptional Daughter. New York: William Morrow, 2015.

Jacob, Jen, and Mardra Sikora. The New Parent's Guide to Down Syndrome: Advice and Information for Raising Your Child. Avon, MA: Adams Media, 2016.


Genetics Home Reference. “Down Syndrome.” National Institutes of Health, U.S. National Library of Medicine. (accessed December 4, 2015).


National Association for Down Syndrome. 1460 Renaissance Dr., Suite #405, Park Ridge, IL 60068. Telephone: 630-325-9112. Website: (accessed December 4, 2015).

National Down Syndrome Society. 666 Broadway, 8th Fl., New York, NY 10012-2317. Toll-free: 800-221-4602. Website: (accessed March 22, 2016).

* genetic (juh-NEH-tik) refers to heredity and the ways in which genes control the development and maintenance of organisms.

* susceptibility (suh-sep-tuh- BIL-uh-tee) means having less resistance to and a greater risk for infection or disease.

* Alzheimer's (ALTS-hy-merz) disease is a condition that leads to gradually worsening loss of mental abilities, including memory, judgment, and abstract thinking, as well as changes in personality.

* correlated means linked in a way that can be measured and predicted. neural

* replicate (REP-luh-kate) means to create an identical copy.

* alpha-fetoprotein (AL-fah FEEtoe- PRO-teen) is a substance produced by a fetus and present in maternal blood and amniotic fluid that is measured to determine the likelihood of neural tube defects.

* amniocentesis (am-nee-o-sen- TEE-sis) is a test during which a long, thin needle is inserted into the mother's uterus to obtain a sample of the amniotic fluid from the sac that surrounds the fetus. The fetal cells in the fluid are then examined for genetic defects.

* umbilical cord (um-BIH-lih-kul) is the flexible cord that connects a baby to the placenta, the organ that unites the unborn child to the mother's uterus, the organ in which the baby develops.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)