A good Q-bank explanation doesn’t just tell you why the right answer is right—it teaches you how to never miss it again and how to eliminate every distractor under pressure. Fragile X is perfect for this because it lives at the intersection of classic genetics, neurodevelopment, and test-writer trap answers (imprinting, trinucleotide repeats, and “just autism”).
Tag: Genetics > Chromosomal Disorders & Syndromes
The Vignette (What You’d See on Test Day)
A 6-year-old boy is brought in for learning difficulties and behavioral problems. He has poor eye contact, hyperactivity, and has been described as “autistic-like.” Physical exam shows long face, large ears, and macroorchidism. His maternal uncle has an intellectual disability.
Question: What is the most likely underlying genetic mechanism?
The Correct Answer: Fragile X Syndrome (FMR1 CGG Repeat Expansion)
The mechanism in one line
CGG trinucleotide repeat expansion in the FMR1 gene (X chromosome) → hypermethylation → transcriptional silencing → ↓ FMRP → abnormal synaptic development.
Key high-yield points
- Inheritance: X-linked, dominant pattern of clinical expression (but with variable penetrance and severity; males usually more severely affected).
- Mutation type: Dynamic mutation (trinucleotide repeat expansion).
- Repeat: CGG on the X chromosome (FMR1, Xq27.3).
- Pathogenesis: Expansion → CpG island hypermethylation → gene silencing → ↓ FMRP (RNA-binding protein involved in synaptic plasticity).
- Anticipation: Yes—typically worsens in successive generations, especially with maternal transmission of a premutation.
- Classic phenotype:
- Intellectual disability (most common inherited cause)
- Autism spectrum behaviors, ADHD
- Long face, large ears, macroorchidism (postpubertal)
- Mitral valve prolapse
- Seizures (can occur)
Premutation pearl (Step 1/2 favorite)
Not everyone with an abnormal repeat count has classic Fragile X:
- Full mutation: typically >200 CGG repeats → methylation + silencing → Fragile X syndrome
- Premutation: ~55–200 repeats → increased FMR1 mRNA (toxic gain-of-function)
- FXTAS (Fragile X–associated tremor/ataxia syndrome): older men, intention tremor, ataxia
- FXPOI (primary ovarian insufficiency): women, early menopause/infertility
Why the Distractors Are Wrong (and How to Recognize Them Fast)
Below are common answer choices that show up alongside Fragile X. Learn the “signature clue” for each.
Distractor 1: Huntington Disease (CAG repeat, gain-of-function)
Why they tempt you: also trinucleotide repeat + anticipation.
Why it’s wrong here: Huntington is CAG expansion on chromosome 4 (HTT gene) causing neurodegeneration, not childhood developmental delay.
Clues for Huntington instead:
- Onset usually 30–50
- Chorea, psychiatric changes, dementia
- Anticipation classically paternal
| Condition | Repeat | Chromosome | Key feature |
|---|---|---|---|
| Fragile X | CGG | X | ID + autism + macroorchidism |
| Huntington | CAG | 4 | Chorea + dementia |
Distractor 2: Myotonic Dystrophy Type 1 (CTG repeat)
Why they tempt you: anticipation + repeat expansion.
Why it’s wrong here: Myotonic dystrophy is primarily a muscle disease with myotonia and multisystem findings, not macroorchidism + autism phenotype.
Clues for myotonic dystrophy instead:
- Grip myotonia (can’t release handshake)
- Distal muscle weakness
- Cataracts
- Cardiac conduction defects
- CTG repeat in DMPK gene
Distractor 3: Prader-Willi Syndrome (Imprinting; paternal deletion or maternal UPD)
Why they tempt you: “chromosomal syndrome” + developmental issues.
Why it’s wrong here: Prader-Willi has a totally different phenotype centered on hypotonia and hyperphagia.
Signature Prader-Willi clues:
- Neonatal hypotonia, poor feeding early → later hyperphagia and obesity
- Hypogonadism
- Mild-moderate ID
- Genetic mechanism: loss of paternal expression on 15q11-q13
- Most commonly paternal deletion
- Or maternal uniparental disomy (UPD)
Distractor 4: Angelman Syndrome (Imprinting; maternal deletion or paternal UPD)
Why they tempt you: also 15q11-q13 imprinting disorder, neurodevelopmental.
Why it’s wrong here: Angelman has a distinct happy demeanor + seizures + ataxia picture.
Signature Angelman clues:
- Severe intellectual disability
- Seizures, ataxia
- Frequent laughter (“happy puppet”)
- Mechanism: loss of maternal UBE3A expression (15q11-q13)
- Usually maternal deletion
- Or paternal UPD
Prader-Willi vs Angelman rapid rule
- Prader-Willi = Paternal deletion
- Angelman = Absent maternal allele
Distractor 5: Klinefelter Syndrome (47,XXY)
Why they tempt you: testicular abnormality in the stem.
Why it’s wrong here: Fragile X causes macroorchidism; Klinefelter causes small, firm testes and hypogonadism.
Clues for Klinefelter instead:
- Tall, long limbs
- Gynecomastia
- Infertility (azoospermia)
- Small testes, low testosterone, ↑ LH/FSH
- Barr body present (one inactivation)
Distractor 6: Down Syndrome (Trisomy 21)
Why they tempt you: common cause of intellectual disability.
Why it’s wrong here: the physical features and family history pattern point away from Down and toward X-linked Fragile X.
Clues for Down instead:
- Upslanting palpebral fissures, epicanthal folds
- Single transverse palmar crease
- Duodenal atresia, AV septal defects
- Early-onset Alzheimer pathology (APP gene on chr 21)
Distractor 7: Turner Syndrome (45,X)
Why they tempt you: “chromosomal disorder” label.
Why it’s wrong here: Turner affects phenotypic females with streak ovaries, not male macroorchidism + autism-like behavior.
Clues for Turner instead:
- Short stature
- Webbed neck
- Coarctation of the aorta (classically)
- Streak ovaries → primary amenorrhea, infertility
The “Why Every Answer Choice Matters” Takeaway: Build an Elimination Algorithm
When you see ID/developmental delay + autism behaviors + long face/large ears + macroorchidism + maternal uncle affected, think:
- X-linked pattern (maternal male relatives)
- Dynamic mutation with anticipation
- CGG repeat → hypermethylation → silencing (loss of function)
If the stem emphasizes hyperphagia/obesity, pivot to Prader-Willi.
If it emphasizes seizures + laughter + ataxia, pivot to Angelman.
If it emphasizes adult-onset chorea, pivot to Huntington.
If it emphasizes myotonia/cataracts, pivot to myotonic dystrophy.
High-Yield Mini Table: Common Repeat Expansions You Must Know
| Disease | Repeat | Core idea | Classic clue |
|---|---|---|---|
| Fragile X | CGG | Hypermethylation → ↓ transcription | Macroorchidism, long face, autism |
| Huntington | CAG | Toxic gain-of-function | Chorea + dementia, paternal anticipation |
| Myotonic dystrophy | CTG | RNA toxicity/splicing issues | Myotonia, cataracts |
| Friedreich ataxia | GAA | ↓ frataxin (mitochondrial) | Ataxia + cardiomyopathy |
Exam-Day Pitfalls (How They Try to Trick You)
- “Autism” alone isn’t the answer. Fragile X is suggested by dysmorphic features + macroorchidism + family history.
- Imprinting vs repeat expansion:
- Imprinting disorders = chromosome 15, parent-of-origin effect, Prader-Willi/Angelman
- Fragile X = X chromosome repeat expansion + methylation
- Testicular size matters:
- Fragile X: macroorchidism
- Klinefelter: small testes
Quick Practice: One-Liner You Should Be Able to Say
“Fragile X is an X-linked trinucleotide CGG repeat expansion in FMR1 causing hypermethylation and gene silencing, leading to intellectual disability, autism features, long face, large ears, and macroorchidism with anticipation.”