Genetic: Down Syndrome
Genetic: Down Syndrome
Down syndrome, or trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21 or by a portion of chromosome 21 translocated (attached) to another chromosome in one of the affected child's parents.
Children with Down syndrome have some degree of mental retardation (average IQ scores are 35–70) as well as characteristic facial features that include a head that is smaller than average, upward-slanting eyes, and a flattened nose. The hands are short and broad with short fingers, and they often have a single crease across the palm. Another characteristic feature of Down syndrome is hypotonia, which is the medical term for poor muscle tone. Children with Down syndrome often need extensive physical therapy in order to learn to walk and move normally. In addition, normal growth is slowed; most of these children never reach full adult height.
Babies with Down syndrome are often born with severe heart defects or blockages of the esophagus and small intestine. These conditions may require surgery shortly after birth. These children are also at increased risk of childhood leukemia.
Adolescents and adults with Down syndrome are more likely than other people to develop health problems that include frequent infections, cataracts, gastrointestinal reflux disease, hearing problems, sleep apnea, dislocated hips, and hypothyroidism.
Down syndrome occurs in about one in every 800 live births in the United States, or about 6,000 children per year. These babies, however, represent only about a quarter of those conceived with trisomy 21. The condition is linked to so many heart defects and other problems that affect survival before birth that about 75 percent of fetuses conceived with Down syndrome are miscarried.
Down syndrome occurs with equal frequency in all races and ethnic groups, as far as is known. Boys are slightly more likely to be affected than girls.
Down syndrome results from genetic errors during the formation of germ cells (eggs and sperm) or during cell division shortly after the egg is fertilized by the sperm. The most common form of Down syndrome, responsible for about 95 percent of cases, occurs when an egg or sperm
carrying two copies of chromosome 21 is involved in conception. The reason for the extra copy in the abnormal germ cell is a genetic error called nondisjunction. During the normal process of germ cell formation, the paired chromosomes in the cell divide so that each daughter cell has only one member of the pair. In nondisjunction, one daughter cell gets both members of the chromosome pair and the other cell has none. If a germ cell carrying two copies of chromosome 21 is fertilized by a normal germ cell from the other parent, the child will have three copies of chromosome 21. This genetic error is called a full trisomy 21.
The Children's Advocate
John Langdon Haydon Down (1828–1896) was the British doctor who pioneered the care and education of children with the syndrome that now bears his name. He published the first scientific description of the condition in 1866. Down had originally hoped to become a chemist and studied under Michael Faraday, one of the most gifted scientists of his time, but then decided to enter medical school in 1853. Down was regarded as exceptionally talented; his professors predicted a brilliant future for him in medicine and were stunned when he turned down a prestigious position at the London Hospital to become superintendent of an asylum for mentally retarded children in Surrey in 1858. At that time, the field of mental retardation was considered unworthy of serious interest or concern, and affected children were regarded as beyond help.
For the next ten years Down worked at the Earls-wood Asylum, turning it into a model institution for the care of mentally ill as well as retarded children and adolescents. In 1868 he founded a school for the education of children with trisomy 21, which was then termed mongolism. Down believed that these children could indeed learn and contribute to society. He also advocated for higher education for women, arguing against the widespread belief that allowing females to study at the university level would make them more likely to have retarded children.
Two of Down's sons became doctors and continued his work at the school he founded. Although the genetic cause of trisomy 21 was not knowninDown'sday—he himself attributed it to tuberculosis in the children'sparents—he was an important advocate for those affected by the syndrome. The cause of Down syndrome was finally identified in 1959 by Jérôme Lejeune (1926–1994), a French pediatrician and geneticist.
Some children with Down syndrome have some body cells with the extra copy of chromosome 21 and some body cells without the extra copy. This condition is called mosaic trisomy 21. It is thought to result from random errors in cell division during the early stages of fetal development. Mosaic trisomy 21 accounts for about 2 percent of children with Down syndrome.
About 3 percent of cases of Down syndrome occur in families. A part of chromosome 21 may become attached to chromosome 14 either before or at the moment of conception. This type of genetic error is called a balanced translocation because there is no extra material from chromosome 21. A person with this type of translocation looks normal and develops normally; however, he or she has an increased risk of having a child with full trisomy 21.
The mother's age is a risk factor for Down syndrome, rising from one chance in 1,562 in mothers age twenty-four or younger to one in nineteen in mothers over age forty-five. Recent studies indicate that the father's age is also a factor; men forty-two years and older are at increased risk of having a child with trisomy 21.
In addition to the physical features mentioned earlier, other indications of Down syndrome in a newborn include:
- An additional skin fold at the inner corner of the eye
- A short neck
- White spots on the iris of the eye known as Brushfield spots
- A round face
- Ears that are smaller than normal
- A flattened area at the back of the head
- Missing teeth or delayed development of teeth
- Protruding tongue and a tendency to breathe through the mouth
- An extra-wide space between the big toe and second toe
Most babies with Down syndrome are diagnosed at birth on the basis of their physical features. The diagnosis can be confirmed by a blood test and karyotype (an analysis of a person's chromosomes).
The diagnosis can also be made before birth. There are two types of prenatal tests: screeners and diagnostic tests. Screeners only estimate the baby's risk of having Down syndrome. Given between the fifteenth and twentieth weeks of pregnancy, screening tests include a blood test and an ultrasound imaging test. These tests are only about 60 percent accurate, however. Diagnostic tests that are about 98 percent accurate include chorionic villus sampling (CVS), done between the ninth and fourteenth weeks of pregnancy; amniocentesis, which can be done at the same time
that the screeners can be given; and testing blood samples taken from the baby's umbilical cord, done after the eighteenth week of pregnancy.
The treatment of children with trisomy 21 is highly individualized. Babies with heart defects or obstructions in the esophagus and digestive tract usually need immediate surgery. Older children require periodic checkups for cataracts, hearing loss, and thyroid problems. Some children need special medications and diuretics for heart problems, and most need to be monitored for frequent infections, particularly ear infections and pneumonia.
At one time children with Down syndrome were either institutionalized or put in special education programs apart from other children. By the early 2000s, however, the emphasis in treatment was to give these children as many opportunities as possible to go to school with other children in their age group and participate in sports, group activities, and other aspects of social life during the growing years.
People with Down syndrome have a shortened life expectancy and a high risk of early Alzheimer disease, often showing a noticeable loss of mental function by age forty. About 85 percent of babies born with trisomy 21 survive the first year of life, but only 50 percent will live to reach age fifty, according to data available in 2008. This is a great improvement, however, as the life expectancy of a person with Down syndrome was only twenty-five years as recently as 1980.
Congenital heart disorders are one reason for the present high mortality rate, as are vulnerability to infections, a high rate of disorders of the digestive tract, and premature aging. Children with Down syndrome are also more likely to develop leukemia than other children.
It is not possible to tell at birth whether a baby with Down syndrome will be severely retarded or will have low-normal intelligence. Individualized assessment of the child is critical to providing opportunities for full development. In general, children with mosaic Down syndrome have higher IQ scores than children with full trisomy 21. There are many adults with the syndrome who are able to hold jobs and live independently; some have become successful artists, actors, and singers.
Since most cases of Down syndrome are caused by a spontaneous genetic mutation rather than an inherited genetic defect, there is no completely effective way to prevent trisomy 21. Adults who are concerned that they may have a balanced translocation of chromosome 21 can choose to have a karyotype to see whether their chromosomes are in fact abnormal. Pregnant women over thirty-five should have tests during the first trimester (three-month period) of pregnancy to screen for the syndrome.
WORDS TO KNOW
Chorionic villus sampling (CVS): A prenatal test that involves taking a small sample of the placenta, the organ that forms inside the uterus during pregnancy and supplies the baby with oxygen and nutrients carried by the blood.
Congenital: Present at birth.
Germ cell: A cell involved in reproduction. In humans the germ cells are the sperm (male) and egg (female). Unlike other cells in the body, germ cells contain only half the standard number of chromosomes.
Hypotonia: The medical term for poor muscle tone.
Karyotype: A photomicrograph of the chromosomes in a single human cell. Making a karyo-type is one way to test for genetic disorders.
Mosaicism: A condition in which a person has some body cells containing an abnormal number of chromosomes and other cells containing the normal number. Mosaicism results from random errors during the process of cell division that follows conception.
Nondisjunction: A genetic error in which one or more pairs of chromosomes fail to separate during the formation of germ cells, with the result that both chromosomes are carried to one daughter cell and none to the other. If an egg or sperm with a paired set of chromosomes is involved in the conception of a child, the child will have three chromosomes in its genetic makeup, two from one parent and one from the other.
Translocation: A genetic error in which a part of one chromosome becomes attached to another chromosome during cell division.
Trisomy: A type of genetic disorder in which a cell contains three copies of a particular chromosome instead of the normal two. Down syndrome is one of several trisomies.
It is possible that the increasing numbers of women having children in their thirties or forties will lead to an increase in the number of children born with Down syndrome. However, as of the early 2000s, about 90 percent of women whose fetuses were diagnosed with the syndrome
chose to end their pregnancies before childbirth. There has been growing concern as to whether such women are being pressured to make this choice. Parents have formed advocacy groups to defend their choice to have their children in spite of the prenatal diagnosis, and disability rights groups have also spoken on their behalf. The first World Down Syndrome Day was held in Singapore in 2006 to raise awareness of the many positive contributions of people with trisomy 21. Sarah Palin, 2008 Republican Vice Presidential candidate, has a son with Down syndrome and raised awareness about children with special needs during her campaign.
Laney, Dawn. Down Syndrome. Detroit, MI: Greenhaven Press, 2008.
Moore-Mallinos, Jennifer. My Friend Has Down Syndrome. Hauppauge, NY: Barrons Educational Series, Inc., 2008.
Skallerup, Susan J., ed. Babies with Down Syndrome: A New Parents' Guide, 3rd ed. Bethesda, MD: Woodbine House, 2008.
Harmon, Amy, and Kassie Bracken. “Reporter's Notebook: An Unusual Campaign.” New York Times, May 2007. Available online at http://video.on.nytimes.com/index.jsp?fr_story=75b21d45037ddde30bf24171111b1ab53bebc333 (accessed August 3, 2008). This is a four-minute video about a campaign to show parents the positive side of rearing a child with Down syndrome.
Nicol, Caitlin. “At Home with Down Syndrome.” New Atlantis 20 (Spring 2008): 143–153. Available online at http://www.thenewatlantis.com/publications/at-home-with-down-syndrome (accessed August 3, 2008). This article is a review of books written by the parents of children with Down syndrome.
Genetics Home Reference. Down Syndrome. Available online at http://ghr.nlm.nih.gov/condition=downsyndrome (updated June 2008; accessed August 3, 2008).
National Down Syndrome Society (NDSS). General Information about Down Syndrome. Available online at http://www1.ndss.org/index.php?option=com_content&task=section&id=23&Itemid=234 (accessed August 3, 2008).
National Human Genome Research Institute (NHGRI). Learning about Down Syndrome. Available online at http://www.genome.gov/19517824 (updated April 10, 2008; accessed August 3, 2008).
National Institute of Child Health and Human Development (NICHD). Facts about Down Syndrome. Available online at http://www.nichd.nih.gov/publications/pubs/downsyndrome.cfm (updated August 18, 2006; accessed August 3, 2008).
"Genetic: Down Syndrome." UXL Encyclopedia of Diseases and Disorders. . Encyclopedia.com. (September 25, 2018). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/genetic-down-syndrome
"Genetic: Down Syndrome." UXL Encyclopedia of Diseases and Disorders. . Retrieved September 25, 2018 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/genetic-down-syndrome
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