This paper examines Down syndrome as a chromosomal abnormality resulting from the presence of an extra chromosome 21. It discusses the three genetic mechanisms that produce the disorder — nondisjunction, mosaicism, and translocation — along with risk factors such as maternal age. The paper covers prenatal screening methods including AFP testing, amniocentesis, and chorionic villus sampling, and describes the physical characteristics used to identify the condition at birth. Associated health complications — including heart defects, gastrointestinal malformations, leukemia risk, and early-onset Alzheimer's disease — are reviewed alongside available treatments. The paper concludes by addressing educational approaches such as classroom mainstreaming and the importance of equitable healthcare and family support for individuals with Down syndrome.
Down syndrome is a chromosomal abnormality accompanied by 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 — 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 — 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 chromosomes instead of 46. In about 25 percent of these occurrences the extra chromosome is contained within the sperm cell, but in the majority of cases it is contained within the egg cell. This may be due in part to the fact that if only a few sperm have been incorrectly produced, the probability that one of the many competing sperm will be that specific defective cell is lower than the probability that a single misproduced egg cell will be fertilized by one of the many sperm attempting to do so. The extra chromosome in Down syndrome is labeled number 21.
It is not inevitable that something be abnormal in one of the reproductive cells of either parent. In only about one to two percent of cases where this extra chromosome occurs, there is nothing abnormal about the original egg and sperm cells that have combined. This is known as a mosaic genetic disorder, where an error occurs after fertilization rather than being carried already in the genetic code of the egg or sperm cells. In these cases, shortly after fertilization there is an abnormal cell division during the period when cells reproduce most rapidly. This abnormal division creates a line of cells that contain the extra chromosome 21. Those who have this mosaic form of Down syndrome will have two types of cells: those with the normal 46 chromosomes and those with the abnormal 47 chromosomes. According to Carson-DeWitt, individuals with mosaic Down syndrome may have fewer symptoms and less severe signs of the disorder than those with the more common type.
A third, and quite rare, genetic occurrence that can cause Down syndrome is called translocation. In these cases, the number 21 chromosome breaks off and becomes attached to another chromosome. The person will actually have the correct total number of chromosomes, but the extra piece of chromosome 21 will cause the symptoms of Down syndrome to occur. This form of the disorder accounts for only three to four percent of cases.
Although it is impossible to completely prevent the occurrence of Down syndrome, several methods are used to help reduce its incidence. One of the first steps taken by a genetic counselor is to ensure that prospective parents are aware that the chances of conceiving a child with Down syndrome increase with age. According to Carson-DeWitt, the chances of conceiving a child with Down syndrome are approximately one in 4,000 during the younger childbearing years. When the woman reaches 35, the risk increases to one in 400 — a very significant increase. By the age of 40, the risk is approximately one in 110, and by age 45 the chances are about one in 35.
Although men with Down syndrome are almost always sterile, women with the condition can often bear children. The chances of a woman with Down syndrome passing the disorder on to her child are approximately fifty percent. In most cases, couples who have had one child with Down syndrome are not significantly more likely to have another child with the condition than any other woman of the same age. However, this is not necessarily true if the Down syndrome was caused by translocation. In that case, it is possible that one parent is a carrier of the genetic defect — even without having the disorder themselves — and the risk of producing offspring with Down syndrome can be calculated once carrier status is established.
"AFP testing, amniocentesis, and CVS procedures"
"Identifying features and common co-occurring conditions"
"Available interventions and life expectancy outlook"
"Mainstreaming, healthcare equity, and family advocacy"
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