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Repeated Sequences - The chance of being affected increases...

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Repeated Sequences Repeated sequences are short segments of DNA that are repeated hundreds or thousands of times. For example: In the segment of DNA illustrated below, CCG is repeated several times. The cause is unknown. Fragile X Syndrome This is the second most common cause of mental retardation (Down Syndrome is first). The characteristic long, narrow face becomes more pronounced with age. The symptoms of fragile-X syndrome appear to be caused by an abnormal number of repeats (CCG) on the X chromosome. Normal DNA has 6 - 50 copies of "CCG" at the locus in question. Carrier males have 50 - 230 copies. This is referred to as a premutation (pre-fragile-X). The full mutation involves more than 230 repeats of CCG.
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Unformatted text preview: The chance of being affected increases in successive generations because extra copies of CCG are added during the gamete-formation process. Females are more likely to add repeats than males. At most, males pass on 230 repeats to their children but females pass on more than 230 repeats. Mental problems are more common if the fragile X is inherited from the mother. This is an example of genomic imprinting discussed in the previous chapter. Fragile-X is more common in males because males inherit their X chromosome from their mother. The repeats cause the X to have a thread-like portion. It is called a fragile site because it breaks if cultured under certain conditions in the laboratory....
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