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 Genetic Defect  

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A 43-year-old man is brought to the general medicine clinic by his wife. She states that his memory has progressively deteriorated over the last several years, and that his personality has been changing. On examination, the physician notes abnormal, writhing movements of the man's limbs and hyperreactive patellar reflexes. An MRI of the head reveals a loss of volume in the neostriatum and cortex. A family history reveals that similar symptoms occurred in several members of the patient's family.

Which of the following genetic mechanisms has been implicated in this disorder?

A. Expanded trinucleotide tandem repeat
B. Genomic imprinting
C. Large deletion in one gene
D. Single amino acid substitution
E. Translocation




C ??








The correct answer is A.

The disease described is Huntington's disease, which is now known to be related to an expanded trinucleotide tandem repeat on the short arm of chromosome 4. The number of trinucleotide repeats frequently increases in succeeding generations during the process of spermatogenesis.

Prader-Willi and Angelman syndromes are frequently cited examples of genetic diseases involving genomic imprinting (choice B).

Duchenne's muscular dystrophy is an example of a disease that is frequently due to a large deletion in a single gene (choice C).

Single amino acid substitutions (choice D) are common in recessive diseases such as sickle cell anemia.

Translocations (choice E) occur in disorders such as chronic myelogenous leukemia (CML; Philadelphia chromosome) and some cases of Down's syndrome.



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