Biology 103
2001 Third Web Report
On Serendip

They used to be called "Mongoloids," an ethnic insult coined by John Langdon Down, an English physician during the nineteenth century. But now they are known as people, individuals with a condition known as Down syndrome. (3). It wasn't until the 1960s that Jerome Lejeune and Patricia Jacobs discovered the cause of Down syndrome (also called trisomy 21). But with technological advancements within the scientific community, more and more information has been gathered about the condition that affects about one in every one thousand children born around the world. (4).

Research shows that Down syndrome is a genetic condition caused by certain chromosomal abnormalities. Chromosomes within cells are composed of proteins and genetic information in the form of DNA. Human cells normally contain 23 pairs of chromosomes to make a total of 46 chromosomes in each cell. When sex cells (eggs and sperm) divide through the process of meiosis, one cell splits into two parts so that each of the resulting cells only has 23 chromosomes, rather than 46. But many errors can occur during cell division. During meiosis, the chromosomes are supposed to split and go to different areas of the cell. This step in the process of meiosis is called disjunction. But sometimes during cell division, a chromosome will not detach and it will stay with its pair chromosome. This results in one of the new cells having 24 chromosomes and the other having only 22 chromosomes. An error such as this is called nondisjunction. If a cell with this error mates with a normal cell, the fertilized egg will end up with an uneven number of chromosomes. (3). In the instance of Down syndrome, 95 percent of all cases are caused by nondisjunction, and 90 percent of all errors occur in the egg cells. In people with Down syndrome, there are three 21st chromosomes due to nondisjunction and the mating of an abnormal cell with a normal cell of 23 chromosomes. This is where the name trisomy 21 originates. (5).

The extra chromosome in trisomy 21 results in overexpression of the genes. Although overexpression is not noticeable in many genes, the genes that are involved in Down syndrome seem to be quite different. In fact, not even all of the genes in the 21st chromosome need to be tripled to result in Down syndrome. There are approximately 250 genes in the 21st chromosome, and only 20 to 50 genes need to be involved in the nondisjunction to cause the effects of trisomy 21. The small area within the cell where genes need to be to cause Down syndrome is called the Critical Region. (3).

There are several genes that researchers believe might be involved in causing Down syndrome. Overexpression in genes such as CAF1A, Cystathione Beta Synthase (CBS), and GART might be harmful to DNA synthesis and repair. COL6A1 overexpression may cause heart defects, and CRYA1 overexpression might aid in the development of cataracts. Overexpression of ETS2 may be the cause of leukemia and skeletal abnormalities, while the DRYK overexpression could possibly result in mental retardation. Premature aging and decreased function of the immune system may be caused by the overexpression of Superoxide Dismutase (SOD1). It is important to note, however, that despite many hypotheses and speculations, no gene has been conclusively linked to causing Down syndrome. (3).

Another type of trisomy 21 is called Robertsonian Translocation. In this instance, two divisions occur in the chromosomes. In many cases, the 21st chromosome replaces some of the 14th chromosome. Although there is still an even number of chromosomes, there is a triplication of the 21st chromosome. When part of the 21st chromosome is translocated to another chromosome it is termed partial trisomy 21. In the case of translocation, it is possible for a child to inherit the abnormal chromosome so parental screening is crucial. Only about three to four percent of all cases of trisomy 21 are due to translocation. (3).

There is also a third type of Down syndrome called mosaicism. Mosaicism, which makes up the rest of all trisomy 21 cases (one to two percent), occurs when a person has a mixture of cell types. Some of these cells have trisomy 21 while others do not. For example, a person could have trisomy 21 in their skin cells, but have normal chromosomes in their blood cells. (4).

It is possible to detect whether or not Down syndrome affects a fetus during pregnancy. There are several different diagnostic and screening methods available to test for trisomy 21. Screening tests are used to detect Down syndrome, but they are not always accurate and must be double-checked with a diagnostic test for confirmation. Maternal alpha-fetoprotein and the triple test are two different options of screening tests. Alpha-fetoprotein tests can detect about 35 percent of Down syndrome cases because of the low level of alpha-fetoprotein that is associated with trisomy 21. The triple test requires samples of the mother's blood levels of unconjugated estriol, human chorionic gonadotropin (hCG), and maternal serum alpha-fetoprotein (MSAFP). A computer to determine the likelihood of having a child with Down syndrome refigures the results of the triple test. The triple test detects approximately 55-60 percent of all Down syndrome cases. (1).

Diagnostic tests include amniocentesis and chorionic villus sampling (CVS). In the amniocentesis, a needle is inserted into the mother's womb to collect fetal cells in the amniotic fluid. The cells then undergo chromosome analysis. The CVS test collects cells from the chorionic villi within the womb that has fetal cells but is not actually the fetus. Both amniocentesis and CVS do carry a small risk of miscarriage. But the tests are highly recommended for mothers over the age of 35 due to the high risk of the occurrence of Down syndrome in children born to older mothers. (1).

There are several effects associated with Down syndrome. Many children born with trisomy 21 have a wide range of mental retardation. Others have mild to severe developmental delay. Heart defects, epilepsy, celiac disease, and hypothyroidism are also prevalent among people with Down syndrome. Respiratory problems, susceptibility to infection, and obstructed digestive tracts are also common among people affected by trisomy 21. Many adults with trisomy 21 develop Alzheimer's disease. Physical effects of Down syndrome often include epicanthal folds over the eyes, flattened bridges of the nose, single palmer creases, and decreased muscle tone. Children with Down syndrome are able to learn to walk, talk, toilet train, and play, but their development is often much slower than their peers. Fluid retention in the ear is common, and speech is often delayed. (6).

Though people born with Down syndrome are often mentally and physically impaired in several ways, they are capable of learning and reacting just the same as other people without the abnormalities associated with the syndrome; emotions, talents, and social capabilities are inherent to people with Down syndrome as well. (2). Though it is still unknown what causes the nondisjunction of the chromosomes in some cells, researchers get closer and closer each year to discovering the mystery of trisomy 21. With every new development, and with increased education and research, the quality of life improves for the people who, not long ago, were exiled from society for being "different" from other people.

WWW Sources

2)"Comprehensive Speech and Language Treatment for Infants, Toddlers, and Children with Down Syndrome",

3)"Trisomy 21: The Story of Down Syndrome",

4)Down Syndrome: Background Information",

5)About Down Syndrome",

6)About Down Syndrome

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