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Down Syndrome ~ Chromosomal abnormality |
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The human body is made of cells; all cells contain a center, called a nucleus, in which genetic material is stored. This genetic material, known as genes, carries the codes responsible for all our inherited characteristics. Down syndrome is usually caused by an error in cell division called non-disjunction. However, two other types of chromosomal abnormalities, mosaicism and translocation, are also implicated in Down syndrome--although to a much lesser extent. Regardless of the type of Down syndrome which a person may have, all people with Down syndrome have an extra, critical portion of the number 21 chromosome present in all, or some, of their cells. |
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Nondisjunction is a faulty cell division which results in an embryo with three number 21 chromosomes instead of two. Mosaicism occurs when nondisjunction of the 21st chromosome takes place in one of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra 21st chromosome. Because of the "mosaic" pattern of the cells, the term mosaicism is used. Mosaicism is rare, being responsible for only 1 to 2 percent of all cases of Down syndrome. Translocation is a different type of chromosomal problem and occurs in only 3 to 4 percent of people with Down syndrome. Translocation occurs when part of the number 21 chromosome breaks off during cell division and attaches to another chromosome. While the total number of chromosomes in the cells remains 46, the presence of an extra part of the number 21 chromosome causes the features of Down syndrome. |
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