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1. What is Down's Syndrome?Down's Syndrome is a genetic condition caused by extra genetic material (genes) from the 21st chromosome. The extra genes cause certain characteristics that we know as Down's syndrome. Individuals with Down's Syndrome also have all the other genes given to them by their parents. As a result, they have a combination of features typical of Down's Syndrome on top of the individual features from their parents. This includes some degree of mental delay, or cognitive challenges and other developmental delays. Some of the physical traits that are common but not always present are epicanthal folds over the eyes, flattened bridge of the nose, a single palmar crease and decreased muscle tone. 2. How did Down's syndrome get its name?Down syndrome gets its name from the British doctor, John Langdon Down, who first clinically identified the condition in 1866. The actual chromosomal nature of Down syndrome did not become documented until 1959 by Dr. Lejeune. However, Down's name remains associated with the condition. 3. What is the cause of Down's Syndrome?It is not clear what causes the extra 21st chromosome. The extra 21st chromosome causes an extra dose of some proteins. Those proteins cause the typical features of Down's Syndrome. 4. What are the types of Down's Syndrome?About 95% of people with Down's syndrome have Trisomy 21, ie. 3 instead of 2 number 21 chromosomes. We normally have 23 pairs of chromosomes, each made up of genes. During the formation of the egg (or the sperm) a woman's (or a man's) pair of chromosomes normally split so that only one chromosome is in each egg (or sperm). In Trisomy 21, or nondisjunction, the twenty-first chromosome pair does not split and a double-dose goes to the egg (or sperm). It is estimated that 95-97% of the extra chromosome is of maternal origin. The second type of Down's Syndrome is known as translocation. It occurs in about 3-4% of people with Down's Syndrome. In this type, an extra part of the twenty-first chromosome gets stuck onto another chromosome. In about half of these situations, one parent carries the extra twenty-first chromosome material in a "balanced", or hidden, form. The third type of Down's Syndrome is called mosaicism. In mosaicism, the person with Down's syndrome has an extra twenty-first chromosome in only some of the cells but not all of them. The other cells have the usual pair of twenty-first chromosomes. One to 2 percent of people with Down's Syndrome have this type. 5. What is the incidence of Down's Syndrome?The incidence of Down's Syndrome has been variously reported as 1 in 800 live births to 1 in 1,100 live births. A recent estimate in the United States puts the incidence at about 1 in 1,000. 6. What are a couple's chances of having a child with Down's Syndrome?In the usual circumstance, the chances depend upon the age of the mother. Many studies were conducted on this and the odds of having a child with Down's Syndrome at age 35 are approximately 1 in 350. Under age 25, the odds are about 1 in 1400 and at age 40 the odds are about 1 in 100. The chances of a parent of a child with Trisomy 21 having another child with Down's Syndrome is approximately 1 in 100. If the child has a translocation, the recurrence risk can be as high as 100% or as low as 2%. Parents of children with translocation type of Down's Syndrome should have chromosome analysis to detect a carrier state. (Pueschel, 1990) 7. Can people with Down's Syndrome have babies?The chance of a woman with Down's Syndrome having a baby with Down's Syndrome is theoretically 50%. There is one documented instance of fertility in a male with Down's Syndrome; the mother also had Down's Syndrome. The pregnancy miscarried about half-way through. 8. When is prenatal diagnosis recommended?Until recently, the answer was to offer amniocentesis to all pregnant women age 35 or older and to those women who had a previous child with Down's syndrome. Now some doctors recommend screening tests for all pregnancies. 9. Are children with Down's Syndrome born only to older mothers?No. Although the risk of Down's Syndrome increases with age, a child with Down's Syndrome can be born to a mother at any age. Although only 5-8% of pregnancies occur in women over the age of 35, they account for 20% of Down's Syndrome births. Therefore, 80% of children with Down's Syndrome are born to women who are less than 35 years of age. |
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