Down Syndrome
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The term Down syndrome is taken from the name of the English physician, Dr. John Langdon Down, who is credited with first describing the condition in 1866. It was not until 1959 that the actual chromosomal abnormality associated with the syndrome was discovered. Dr. Jerome Lejuene found that individuals with Down syndrome possessed additional genetic material in their cells, usually an extra chromosome. Instead of having 46 chromosomes in each cell (22 pair of autosomes or non-sex chromosomes and one pair of sex chromosomes, xx in females, xy in males), individuals with Down syndrome most commonly have 47 chromosomes with the extra chromosome associated with the 21st pair. The term Trisomy 21 is therefore used to describe this configuration of three #21 chromosomes. About 95% of all individuals with Down syndrome have Trisomy 21. 37
Ninety-five percent of people with Down syndrome (trisomy 21) have 47 chromosomes per cell (they have an extra #21 chromosome). This common type of trisomy 21 is called non-disjunction. 27
this occurs in 3-4% of cases of Down syndrome. Children with translocation have an extra #21 chromosome that is attached to another chromosome, usually a #14 or another 21. About 2/3 of translocations occur spontaneously during fertilization. In the remaining 1/3, either parent can carry a balanced translocation which means they still have 46 chromosomes but two of them are attached (21/21 or 21/14). When a child with Down syndrome has translocation it is important the parent�s chromosomes be studied to determine if one of them is a �carrier� of the translocation. This can have important implications for genetic counseling. 10
Chromosomes hold the body�s genetic blueprint. Most cells in the body contain 22 pairs of chromosomes and a 23rd set of either XX (in females) or XY (in males) for a total of 46 chromosomes. Reproductive cells, eggs and sperm, contain a single set of 23 chromosomes that combine when an egg is fertilized to form a new set of 46 in the fetus (half from each parent). In most cases of Down syndrome, random chance leads to the insertion of an extra copy of chromosome 21 in either the egg or sperm. This extra copy becomes part of the fertilized egg and is replicated in all of the embryo�s cells. This form of Down syndrome is called trisomy 21, and it accounts for about 95% of DS cases. 43
In Trisomy 21, the extra chromosome can usually be traced to the egg, although it has been hypothesized that it may come from the sperm in 20% of the cases. It is suspected that during cell division of the egg (or sperm), the two #21 chromosomes do not separate properly and one cell will therefore have an extra #21 chromosome which later, if united with a normal cell, may lead to Trisomy 21. What causes this improper cell division is unknown. Factors such as viral infections hormonal abnormalities, x-rays, and certain drugs are being investigated as potential causal factors. A small number of cases are genetic in origin; these can be determined through genetic counseling. 37
Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome. Although the connection between Down syndrome and Alzheimer disease is unclear, researchers believe that an extra copy of a particular gene on chromosome 21, the APP gene, may account for the increased risk. 4
A normal human cell contains 23 pairs of chromosomes which carry all of a person’s genetic information. Due to several possible abnormal mechanisms of cell reproduction, patients with Down syndrome have an extra (third) copy of the 21st chromosome. Thus, Down syndrome is also called Trisomy 21. 46
It is also possible that a child with mosaic Down syndrome inherited a total of 46 chromosomes at the time of conception. If this were the case, then the error in chromosomal separation, which resulted in the formation of a second cell grouping with an extra chromosome #21, occurred early in the baby’s development. 14
Down syndrome is a genetic disorder that causes mental retardation and physical defects. It is caused by the presence of an extra copy of chromosome number 21. This condition is called trisomy 21. 8
Down syndrome is a relatively common birth defect caused by the presence of an extra chromosome number 21 (three instead of two number 21 chromosomes, or, trisomy 21). This chromosome abnormality adversely affects both the physical and intellectual development of the individual. 11
Most cases of Down syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 21 in each of the body’s cells. 4
Down syndrome is the most common of the forms of mental retardation that can be identified during the prenatal and perinatal periods. Amniocentesis, which involves inserting a needle through the mother’s abdomen and withdrawing amniotic fluid (fluid surrounding the fetus), can be performed around the 14th week of pregnancy. The fluid can be subjected to chemical tests and the chromosomes analyzed. In this manner, Down syndrome can be detected. Amniocentesis is often recommended if there is a history of Down syndrome in the family or if the woman is over 35 years of age. Alternatives to amniocentesis such as chorionic villus sampling are currently being investigated. 37
Translocation Down syndrome can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition. 4
The chromosome abnormality that causes Down syndrome is trisomy 21, an extra copy of chromosome number 21. This means that instead of having the normal 2 copies of chromosome number 21, the person with Down syndrome has 3 copies of chromosome number 21. Confirmation requires a chromosome study (analysis under the microscope of the chromosomes). 11
Down’s syndrome occurs in about 1 out of every 800 to 1000 births. It affects an equal number of females and males. The most common cause of Down’s syndrome is an extra #21 chromosome in the mother’s egg. As a woman grows older, she faces a greater risk of having a baby with Down’s syndrome. For example, a woman in her twenties faces a risk of about 1 in 4,000. By the age of thirty-five, that risk has increased to 1 in 400 and, by the age of forty-five, to 1 in 35. 41
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