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Discussion Starter · #1 ·
A family is referred to a genetic specialist because of mild mental retardation in several of the school-age children. Laboratory evaluation demonstrates a specific chromosomal breakage site in metaphase studies of lymphocytes cultured with methotrexate. Which of the following chromosomes is most likely to be specifically affected?
A. X
B. Y
C. 13
D. 16
E. 21
 

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Discussion Starter · #5 ·
Choice A.

The heritable mental retardation condition associated with chromosomal breakage is fragile X syndrome. The break point is at q27.3 on the X chromosome. The test for this condition involves culturing lymphocytes in either folate-deficient medium or with chemical agents such as methotrexate that tend to break chromosomes. To make the diagnosis, at least 4% of metaphase chromosomes must show the specific break-point on the X chromosome.
 

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I have a doubt regarding this. Dosn't methotrexate disrupts all DNA or chromosome, as we use to inhibit cell replication in cancer cells? So why only X chromose show breakage? Can any one clarify? I know this is a very old thread, just hoping i'd get an explanation.
 
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