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Download Trisomy 21: The Story of Down Syndrome What is Down syndrome?
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Transcript
Trisomy 21: The Story of Down Syndrome What is Down syndrome? Down syndrome is a set of physical and mental traits caused by a gene problem that happens before birth. Children with Down syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of mental retardation. This varies from person to person, but in most cases it is mild to moderate. Down syndrome is a lifelong condition. But with care and support, most children with Down syndrome can grow up to have healthy, happy, productive lives. The Chromosomes Chromosomes are thread-like structures composed of DNA and other proteins. They are present in every cell of the body and carry the genetic information needed for that cell to develop. Genes, which are units of information, are "encoded" in the DNA. Human cells normally have 46 chromosomes which can be arranged in 23 pairs. Of these 23, 22 are alike in males and females; these are called the "autosomes." The 23rd pair are the sex chromosomes ('X' and 'Y'). Each member of a pair of chromosomes carries the same information, in that the same genes are in the same spots on the chromosome. However, variations of that gene ("alleles") may be present. (Example: the genetic information for eye color is a "gene;" the variations for blue, green, etc. are the "alleles.") Human cells divide in two ways. The first is ordinary cell division ("mitosis"), by which the body grows. In this method, one cell becomes two cells which have the exact same number and type of chromosomes as the parent cell. The second method of cell division occurs in the ovaries and testicles ("meiosis") and consists of one cell splitting into two, with the resulting cells having half the number of chromosomes of the parent cell. So, normal eggs and sperm cells only have 23 chromosomes instead of 46.This is what a normal set of chromosomes looks like. Note the 22 evenly paired chromosomes plus the sex chromosomes. The XX means that this person is a female. The test in which blood or skin samples are checked for the number and type of chromosomes is called a karyotype, and the results look like the picture above. Many errors can occur during cell division. In meiosis, the pairs of chromosomes are supposed to split up and go to different spots in the dividing cell; this event is called "disjunction." However, occasionally one pair doesn't divide, and the whole pair goes to one spot. This means that in the resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes. This accident is called "nondisjunction." If a sperm or egg with an abnormal number of chromosomes merges with a normal mate, the resulting fertilized egg will have an abnormal number of chromosomes. In Down syndrome, 95% of all cases are caused by this event: one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21. Recent research has shown that in these cases, approximately 90% of the abnormal cells are the eggs. The cause of the nondisjunction error isn't known, but there is definitely connection with maternal age. Research is currently aimed at trying to determine the cause and timing of the nondisjunction event. Analysis Questions 1. Draw a nondisjunction in anaphase 1 or 2 during meiosis. Assume the cell starts with two pairs of chromosomes before replication. Frayer Model 2. Create a Frayer Model for the word chromosome. 3. Explain how trisomy 21 (Down Syndrome) is identified with a karyotype. 4. Analyze the two karyotypes above to determine the sex of each individual. Extension 5. Turner syndrome is a condition where females have one x chromosome instead of two. Explain the mechanism or cause of Turner Syndrome. 6. Edwards syndrome (trisomy 18), Patau Syndrome (trisomy 13), Turner Syndrome and Down Syndrome (trisomy 21) are the only disease caused by a nondisjunction. Predict the impact of a nondisjunction of other chromosomes in a sperm or egg that is fertilized.