Down Syndrome

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Patricia & Lay-Ying
July 20, 2004
Genoveva Mayer
English 122
Trisomy 21: A Genetic Disease
Down Syndrome is the most common genetic disorder caused by an abnormal cell division of the chromosome 21. Unfortunately, it occurs in one out of eight hundred live births, no matter the race or the economic class of the individuals. There are sometests available in order to detect the three different types of DS. Nevertheless, there is no way to prevent it, but there are some factors that indicate its possible appearance.
In 1866, Dr John Langdon Down Syndrome discovered DS. He used to work with mentally retarded patients and he realized that some of them had the same common features. He named them “Mongoloids” because they lookedlike people from Mongolia. However, it is a racist concept because it is offensive for many Asian people. In 1959, Jerome Lejeune and Patricia Jacobs determined that DS was caused by an abnormal cell division of a trisomy of chromosome 21(Leshin, pars. 1-3).
First, we must determine the structure and the function of human chromosomes in order to understand why DS occurs. Chromosomes arestructures that are in charge of transmitting the genetic information needed for that cell to develop. Normally, human cells have 23 pairs of chromosomes. Of these 23, 22 are allocated for the sperm cells in males and for the ovum in females. The 23rd chromosome determines the sex of the baby(Leshin, par.4).
According to Len Leshin in his article “Trisomy 21: The Story of Down Syndrome”,there are two ways of human cell division. One of them is called “mitosis” in which one
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cell becomes two cells that are exactly the same as the parent cell. The second method is “meiosis”. It happens during the cell division of egg or sperm cell production, in which reproduction cells split up into two which have half the number of chromosomes of the parent cell.Nevertheless, many genetic variations may occur during cell division causing one of the three different types of DS such as Standard Trisomy 21, Translocation and Mosaicism.
Standard Trisomy 21 is the most common one. It occurs when one pair of chromosomes fails to separate and both chromosomes go to one side of the dividing cell. This cell division during meiosis is called non-disjunction. Despitemany research, no one knows exactly what factors cause non-disjunction (Taylor, p. 626).
Another type of Down syndrome is Robertsonian Translocation. It occurs when part of the chromosome 21 breaks off during meiotic cell division and gets attached usually onto the chromosome number 14. In some cases, “children would have a triplication of part of the 21st chromosome instead of the wholechromosome which is called partial trisomy 21”. Translocation can be inherited by one of the parents. That is why they should make a karyotype test in order to see if either of them may be a carrier. If neither of them is a carrier, the DS is a result of a faulty cell division (1: 627).
The least common type of DS is Mosaic Down Syndrome or Mosaicism.
Those affected have a mixture of celltypes in their bodies. Some cells are normal and contain 46 chromosomes, while others contain 47, carrying an extra copy of chromosome 21(1: 627).
According to the statistics shown in the book Atlas of Perinathology, the possibilities to have a child with DS when the mother is carrier is about 12% while when the father is a carrier is 8%. Another data shown in the book is that 94% of thecases of DS
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is Standard Trisomy 21, followed by 4% of the cases by translocation and the least common one is mosaicism with only 2%(1: 452).
The presence of an extra set of genes in trisomy 21 is called “overexpression” and can produce certain characteristic features of DS. The 21st chromosome may have 200 to 250 genes, which some of them causes some characteristics...