Chromosomes are structures that carry genetic information in cells. They carry the instructions that tell cells what functions they are to perform. They determine the way a person's body looks and the way it functions.
Cells normally carry two matched sets of twenty-three chromosomes for a total of forty-six chromosomes. One set of twenty-three chromosomes comes from each parent. Down's syndrome occurs when one chromosome pair is damaged. That pair is designated as chromosome #21.
Chromosome #21 can be damaged, for example, if one parent has two chromosomes at location 21, rather than one. A child will then get two #21 chromosomes from one parent and one #21 chromosome from the other parent, for a total of three #21 chromosomes. This form of Down's syndrome is called trisomy 21, meaning that the #21 chromosome has three units rather than two. Overall, the child has forty-seven chromosomes, rather than forty-six chromosomes. This pattern of changes occurs in more than 90 percent of all Down's patients.
Chromosome #21 can also be damaged during cell replication. Cells grow and reproduce over time. As they do so, they make copies of the original chromosomes from the parents. Sometimes, the chromosomes from the parents are normal but a problem occurs when the new cells are reproducing. Two normal chromosomes at location 21 become three chromosomes in some cells.
This condition is known as a mosaic disorder. People with this disorder have some cells containing forty-six chromosomes, and some containing forty-seven chromosomes. They may have less severe symptoms than a person whose cells all contain forty-seven chromosomes. A mosaic condition occurs rarely. Less than 2 percent of all Down's cases are caused by mosaic disorder.
Chromosome #21 can also be damaged when a normal chromosome from the parent breaks into two pieces. One piece of the chromosome becomes attached to another chromosome. This process is called translocation. Translocation occurs in about 3 to 4 percent of all Down's patients.
Down's Syndrome: Words to Know
- A structure located inside the nucleus (center) of a cell that carries genetic information.
- The specific chromosomal makeup of a particular cell.
- Mental retardation:
- A condition in which an individual has a lower-than-normal IQ.
- Medically, a condition in which an individual cell may contain more than one type of chromosomal composition, with forty-six chromosomes in one cell, for example, and forty-seven chromosomes in another cell.
- A condition in which a piece of one chromosome breaks off and becomes attached to another chromosome.
- A condition in which three identical chromosomes, rather than two, are matched with each other.
Down's syndrome occurs in about 1 out of every 800 to 1000 births. It affects an equal number of females and males. The most common cause of Down's syndrome is an extra #21 chromosome in the mother's egg. As a woman grows older, she faces a greater risk of having a baby with Down's syndrome. For example, a woman in her twenties faces a risk of about 1 in 4,000. By the age of thirty-five, that risk has increased to 1 in 400 and, by the age of forty-five, to 1 in 35.
Babies with Down's syndrome can often be diagnosed at birth because of some common physical characteristics. For example, they tend to be unusually quiet, less responsive, and weak. Other physical signs include:
- Flat face
- Small head
- Flat bridge of the nose
- Smaller than normal, low-set nose
- Small mouth, causing the tongue to stick out and look unusually large
- Upward slanting eyes
- Extra folds of skin at the inside corner of each eye
- Rounded cheeks
- Small, misshapened ears
- Small, wide hands
- A deep crease across the center of each palm
- A malformed fifth finger
- A wide space between the big and second toes
- Unusual creases on the soles of the feet
- Overly-flexible joints (as in people who are double-jointed)
- Shorter than normal height
Other physical defects may accompany Down's syndrome. About 30 to 50 percent of children with the disorder have heart problems. These defects often reduce the flow of blood through the body.
Gastrointestinal (digestive) tract disorders affect about 5 to 7 percent of all children with Down's syndrome. The most common problem among babies is an obstruction (narrowing or closing) of the upper part of the intestine, where digestion takes place. This obstruction can interrupt the digestion of food, and a baby may vomit after eating. As a result, the baby has trouble gaining weight appropriately.
Other medical conditions that may accompany Down's syndrome include an increased risk for infections, especially ear infections (see earache entry) and pneumonia (see pneumonia entry); certain kidney disorders; thyroid disease; hearing loss (see hearing loss entry); vision problems (see vision disorders entry); and a greatly increased risk for leukemia (see leukemia entry).
Physical development overall takes place at a slower rate in babies with Down's syndrome than in normal babies. Down's babies tend to have weaker muscles, so they have more difficulty learning to sit up, crawl, and walk. Mild to moderate mental retardation is also common in Down's children. The range of IQs (a measure of intelligence) of Down's children varies widely. Some have normal intelligence, while others have severe mental retardation. Most fall within a range sometimes called the trainable range. That means that individuals can be taught to care for themselves and to function normally in social settings and even have simple jobs.
As people with Down's syndrome age, they face an increased risk for Alzheimer's disease (see alzheimer's disease entry). The risk of developing Alzheimer's disease in the general population is about 6 in 100. Among people with Down's syndrome, the risk is 25 in 100. Aging also increases the risk of other conditions for people with Down's syndrome, including cataracts (see cataracts entry), thyroid problems, diabetes (see diabetes mellitus entry), and seizure disorders (see epilepsy entry).
Diagnosis is often made at birth, when the characteristic physical signs of Down's syndrome are noted. Genetic tests can then confirm the diagnosis. These tests look for abnormal chromosomes. A blood or skin sample is taken from the child. The sample is then stained with a special chemical that makes the chromosomes visible. The pattern of chromosomes that shows up is known as a karyotype (pronounced KARR-ee-uh-tipe). The presence of an abnormal chromosome #21 stands out in the karyotype.
There is no cure for Down's syndrome. Many of the conditions that occur along with the disorder, however, can be treated. For example, surgery can be done to repair heart defects. Eyeglasses and hearing aids can help with vision and hearing problems.
Years ago, Down's syndrome children were placed in special institutions early in their lives. They usually spent their whole lives there. We now know that most children with Down's syndrome are able to live at home with their families. With training, most families can provide the care children with Down's syndrome need to lead relatively normal lives.
Some special community groups help families adjust to the presence of a new Down's syndrome child. They can help parents and siblings (brothers and sisters) understand the disorder and teach them how to plan for the baby's future. Schools are required by law to provide services for children with Down's syndrome. Sometimes these services are offered in special classes, and sometimes in regular classrooms. The practice of teaching children with Down's syndrome and other disabilities in regular classrooms is called mainstreaming or inclusion.
The prognosis in Down's syndrome is quite variable. It depends on the other disorders that occur in each individual case. A child with heart defects, for example, is more likely to have serious problems in the future than one with hearing loss. About 90 percent of all Down's syndrome children with no heart problems reach their teens.
People with Down's syndrome usually go through the same physical changes of aging as other people. But those changes take place more rapidly. The average age of death for a person with Down's syndrome is about fifty to fifty-five years.
The prognosis for a baby born with Down's syndrome is better now than ever before. Antibiotics are now available to treat many kinds of infections that would once have killed a child. Community and family support allow people with Down's syndrome to live more normal lives. Down's syndrome people are often able to get an education and hold jobs.
Men with Down's syndrome appear to be uniformly sterile. That is, they are unable to have children. Women with Down's syndrome, however, are usually able to have babies. About half of these babies will also be born with Down's syndrome.
The only procedure available for preventing Down's syndrome is genetic counseling. In genetic counseling, a trained professional talks with a man and a woman about the chances of their having a Down's syndrome baby. The counselor can inform a couple, for example, that the risk for Down's syndrome increases with the mother's age. Certain tests can also be performed to help with the counseling program.
One of these tests makes use of a protein known as alpha-fetoprotein (AFP, pronounced AL-fuh FEE-toe-PRO-teen). AFP occurs normally in the blood of a pregnant woman. Unusually high or low levels of AFP can, however, suggest problems with the pregnancy. An AFP test can accurately predict the presence of a Down's syndrome baby about 60 percent of the time.
Two other tests are far more accurate. These involve the use of amniocentesis (pronounced AM-nee-oh-sen-TEE-sus) or chorionic villus sampling (CVS, pronounced kore-ee-AH-nik VI-lus). Both are correct about 98 to 99 percent of the time in predicting the presence of a Down's syndrome baby.
In amniocentesis, a long, thin needle is used to remove a sample of amniotic fluid. Amniotic fluid is the liquid that surrounds and protects the baby developing within the pregnant woman. In CVS, a tiny tube is used to remove a small sample of the placenta. The placenta is a spongy tissue that provides nourishment for the growing baby.
In both tests, the samples obtained can be used to make a karyotype of the baby's chromosomes. The karyotype can be studied to see if the chromosomes in position #21 are abnormal. If they are, there is a very strong probability that the baby will be born with Down's syndrome.
A genetic counselor can do no more than provide information to parents about the chance that their baby might be born with Down's syndrome. After that, parents have to make their own decision as to what to do with this information. Some parents decide to have the baby and give it the best possible care. Other parents decide not to continue with the pregnancy.
Once a couple has had one baby with Down's syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. In most cases, the chances of having a second Down's syndrome child are about the same as having the first. In certain forms of the disorder, however, the chance may increase. For example, if the disorder was caused in the first case by a translocation, the chance of having a second Down's syndrome child is much higher. Genetic counselors are able to calculate with some precision the risk in such cases.
FOR MORE INFORMATION
Levitz, Mitchell, and Andy Bricky. Count Us in: Growing Up With Down Syndrome. New York: Harcourt Brace & Co., 1994.
Pueschel, Siegfried M., ed. Parents Guide to Down Syndrome: Toward a Brighter Future. Baltimore: Paul H. Brookes Publishing Company, 1995
Pueschel, Siegfried M., and Maria Sustrova, eds. Adolescents With Down Syndrome: Toward a More Fulfilling Life. Baltimore: Paul H. Brookes Publishing Company, 1997.
Stray-Gunderson, K. Babies with Down Syndrome: A New Parents' Guide, 2nd edition. Bethesda, MD: Woodbine House, 1995.
Tingey, C. Down Syndrome: A Resource Handbook. Boston: Little, Brown, Inc., 1988.
"Medical and Surgical Care for Children with Down Syndrome." The Exceptional Parent (November 1995): pp. 78+.
Roan, Shari. "Elixir of Hope?" Los Angeles Times (March 6, 1996): p. E1.
National Down Syndrome Congress. 1605 Chantilly Drive, Suite 250, Atlanta, GA 30324–3269. (800) 232–6372.
"Ask NOAH About: Mental Health." NOAH: New York Online Access to Health. [Online] http://www.noah.cuny.edu/mentalhealth/mental.html#MentalRetardation (accessed on October 25, 1999).
www.downs-syndrome.org.uk Website of the Down's Syndrome Trust