Question:medium
Which of the following is true about Fragile X syndrome?
Which of the following is true about Fragile X syndrome?
Show Hint
It is a CGG repeat loss-of-function disorder; fully affected males have IQ around 40.
Updated On: Jun 23, 2026
- Triple nucleotide CAG sequence mutation
- 10% female carriers are mentally retarded
- Males have IQ 20-40
- Gain of function mutation
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: Fragile X is an X-linked disorder driven by an unstable CGG repeat in FMR1 on the long arm of the X chromosome. Inheritance does not follow simple Mendelian rules because of progressive allelic (repeat) expansion across generations.
Step 2: A full mutation methylates and switches off FMR1, abolishing FMRP. This is a loss-of-function mechanism, so any "gain of function" statement is incorrect.
Step 3: The expanded triplet is $CGG$, distinguishing it from the $CAG$ expansions of Huntington disease, so the CAG option fails.
Step 4: Clinically, fully affected males show macroorchidism, a long face with large ears and a prominent jaw, and marked intellectual disability with a mean IQ near $40$. The true statement is that males have an IQ in the $20$ to $40$ range.
\[\boxed{\text{Males have IQ 20-40}}\]
Step 2: A full mutation methylates and switches off FMR1, abolishing FMRP. This is a loss-of-function mechanism, so any "gain of function" statement is incorrect.
Step 3: The expanded triplet is $CGG$, distinguishing it from the $CAG$ expansions of Huntington disease, so the CAG option fails.
Step 4: Clinically, fully affected males show macroorchidism, a long face with large ears and a prominent jaw, and marked intellectual disability with a mean IQ near $40$. The true statement is that males have an IQ in the $20$ to $40$ range.
\[\boxed{\text{Males have IQ 20-40}}\]
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