**Question:** The over-expression of a trinucleotide repeat causes

A. Huntington's disease
B. Myotonic dystrophy
C. Friedreich's ataxia
D. Tourette's syndrome

**Correct Answer:** .

**Core Concept:** Trinucleotide repeats are short segments of DNA consisting of three nucleotides (adenine, guanine, cytosine, and thymine) repeated multiple times. Over-expression or expansion of these repeats can lead to genetic disorders.

**Why the Correct Answer is Right:** Over-expression of trinucleotide repeats can lead to genetic disorders due to the instability of these repeats during cell division. When the number of repeats expands, the affected gene may lose its ability to function properly, leading to the development of a genetic disorder.

**Why Each Wrong Option is Incorrect:**

A. Huntington's disease is caused by an expansion of CAG trinucleotide repeats within the HTT gene, leading to the loss of function and neuronal dysfunction.

B. Myotonic dystrophy is caused by an expansion of CTG trinucleotide repeats within the DMPK gene, leading to a decrease in protein function and muscle dysfunction.

C. Friedreich's ataxia is caused by an expansion of GAA trinucleotide repeats within the FXN gene, leading to a decrease in frataxin protein production and mitochondrial dysfunction.

D. Tourette's syndrome is caused by an expansion of CAG trinucleotide repeats within the CHRNA7 gene, leading to the loss of function of alpha7 nicotinic acetylcholine receptors and neurological symptoms.

**Clinical Pearl:** Trinucleotide repeat disorders are typically caused by the expansion of these repeats, leading to a loss of gene function or reduced protein production, which results in various clinical manifestations. Understanding the molecular basis of these disorders helps in diagnosing and counseling patients and families affected by these conditions.