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Chromosomal Abnormality: Down Syndrome
Down Syndrome is a chromosomal abnormality which is accompanied by both serious physical and mental developmental problems, and it is one of the most common genetic syndromes. According to Rebecca Saenz (1999), Down Syndrome occurs in one out of every 800 to 1,000 live births, which is an extremely high rate of incidence. This disorder occurs equally among both boys and girls. There are several genetic occurrences that can cause Down Syndrome to appear in a child, but all of them lead to the same chromosomal abnormality. An extra chromosome, which is the unit of genetic information that exists within each cell, appears in the genetic code of the affected person. When a baby is normally conceived, the egg cell of the mother and the sperm cell of the father each contribute 23 chromosomes to the genetic makeup of the child. In some cases, there may be an error in the production of that egg cell or sperm cell, and it will contain 24 chromosomes instead of the normal 23, leading to a baby with a total of 47, instead of 46, chromosomes. In about 25% of these occurrences, the extra chromosome will be contained within the sperm cell, but in the majority of cases this chromosome will be contained within the egg cell. (This may be due in part to the fact that if only a few sperm have been misproduced, the chances that out of the many sperm competing to fertilize the egg cell it will in fact be that specific sperm cell that succeeds is lower than the chances that one single misproduced egg cell will in fact be fertilized by the many sperm attempting to do so.) The extra chromosome in Down syndrome is labeled number 21.
However, it is not inherent that something be abnormal in on of the reproductive cells of either in the mother or the father. In only about one to two percent of cases where this extra chromosome occurs, there is nothing abnormal whatsoever about the original egg and sperm cells which have combined. This would be a mosaic genetic disorder, where an error occurs after fertilization, as opposed to being carried already in the genetic code of the egg or sperm cells. In these cases, shortly after fertilization, there will be an abnormal cell division during the time when cells reproduce the most rapidly of any point in development. This abnormal cell division will create a line of cells which contain the extra chromosome 21. Those who have this mosaic form of Down syndrome will actually have two types of cells, including both those with the normal 46 chromosomes, and also those with the abnormal 47 chromosomes. According to Carson-DeWitt, it has been observed that individuals with mosaic Down syndrome may have fewer symptoms and less severe signs of the disorder than those with the more common type of Down syndrome.
A third, and quite rare, genetic accident which can cause Down syndrome is called translocation. In these cases, the number 21 chromosome somehow breaks off and becomes attached to another chromosome. In these cases, the person will actually have the correct number of chromosomes, but the extra piece of chromosome 21 will cause the symptoms of Down syndrome to occur. This form of the disorder only occurs in three to four percent of the cases.
Although it is impossible to completely prevent the occurrence of Down syndrome, there are several methods which are meant to assist in the efforts to prevent as many occurrences as possible. The first efforts made by a genetic counselor in the prevention of Down syndrome is to make sure that the couple is aware that the chances of conceiving a child with Down syndrome increases with age. According to Carson-DeWitt, the chances of conceiving a child with Down syndrome is approximately on in 4,000 during the younger childbearing years. When the woman reaches 35 years old, the risk has increased to one in 400, a very significant leap. By the age of 40 the risk is approximately one in 110, and by the age of 45 the chances are about one in 35. Although men with Down syndrome are almost always sterile, women can often bear children. The chances of a woman with Down syndrome passing the disorder on to her child is approximately fifty percent. Usually, couples who have had one child with Down syndrome are not any more likely to have another child with Down syndrome than any other woman of the same age. However, this is not necessarily true if the Down syndrome is caused by the more rare translocation occurrence. In this case, it is actually possible that one parent is a carrier of the genetic defect, although he or she does not actually have the disorder, and it is possible to calculate the risk of producing offspring with Down syndrome if it is known that one parent is actually a carrier of this.
Pre-birth screening methods for Down syndrome are dominated by two basic forms of testing. According to Carson-DeWitt, the first is done at approximately 14 to 17 weeks of pregnancy, when alpha-fetoprotein (AFP) levels can be measured from the blood of the pregnant woman. Levels of AFP are usually lower if the woman is carrying a child with Down syndrome. Two other hormones are also measured. These chemical levels are calculated in with the mother's age to determine the actual risk that the child will be born with Down syndrome. These results are only accurate approximately 60% of the time, which is only slightly better than guessing. A far more accurate procedure can be performed slightly further into the pregnancy, which is actually testing a tissue sample from around the unborn child. Two ways of collecting this tissue are through amniocentesis or through chorionic villus sampling (CVS). In the first, amniotic fluid from within the womb is collected using a long needle inserted through the skin of the mother. In the second, a sample of the placenta is collected through the vaginal opening. According to Saenz (1999), women over 35 are at a great enough risk for producing a child with Down syndrome that this test is recommended. This test is considered to be about 98 to 99% accurate, and many women use the results of this test as reason to terminate the pregnancy, rather than simply using it to prepare for the arrival of a special-needs child.
After birth, a Down syndrome child is usually detected because of the occurrence of common physical characteristics, or strange infant behavior. Carson-DeWitt writes that babies with this disorder are very quiet and less responsive to stimuli, and that the muscles may be weak or floppy. However, the most noticeable signs are the physical characteristics, which may include a flat appearing face, small head, flat bridge of the nose, smaller than normal or low-set nose, small mouth, upward slanting eyes, extra folds of skin located at the inside corner of each eye, rounded cheeks, small and misshapen ears, small and wide hands, an unusual deep crease across the center of the palm (called a simian crease), a malformed fifth finger, a wide space between the big and the second toes, unusual creases on the soles of the feet, overly-flexible joints, and/or shorter than normal height. (Carson-DeWitt) If these signs are observed, a blood test to confirm the chromosomal anomaly is performed.
People affected by Down syndrome are likely to have many other disorders and health problems. Between 30 to 50% of those with Down syndrome will have heart defects, including the possibility of holes between the heart chambers, or a hole leading to the outside of the heart, both of which will interrupt blood flow and limit oxygen circulation.
Between five to seven percent of those with Down syndrome are found to have malformations of the gastrointestinal tract, which may include a narrowed duodenum which connects the stomach to the small intestine, which may prevent infants from digesting milk and cause forcible vomiting after each feeding. Carson-DeWitt writes that patients with Down syndrome also have an increased chance of developing infections, especially ear infections and pneumonia, certain kidney disorders, thyroid disease, hearing loss, vision impairment, and a 20-times greater chance of developing leukemia. Those with Down syndrome are also more likely to develop Alzheimer's as they age. Of course, the most common condition caused by Down syndrome is the delayed development of crawling, walking, and talking. The IQ range of Down syndrome patients is quite varied, and most people with Down syndrome are able to care for themselves throughout most of their lives.
Although there is no cure for Down syndrome, there are many treatments for the individual problems that may arise. Surgery can be performed to repair heart or gastrointestinal disorders. Glasses can be worn to help counter the visual impairments, and hearing aids can assist those suffering from hearing loss. Carson-DeWitt writes that the prognosis of a child with Down syndrome is quite varied…[continue]
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