Q-Bank Breakdown: Anticipation — Why Every Answer Choice Matters
Tag: Genetics > Clinical Genetics
Anticipation is a classic test-writer favorite in clinical genetics because it’s both high-yield and clinically intuitive: some inherited disorders get worse or present earlier in successive generations. In q-banks, the key is not just spotting the correct diagnosis—but also understanding why each distractor is wrong.
Clinical Vignette (USMLE-Style)
A 41-year-old man is evaluated for progressive clumsiness and involuntary movements. Over the past year, he has developed irritability and memory problems. His father died at age 62 after a long course of similar symptoms. The patient’s 16-year-old daughter has recently developed declining school performance and mood changes. Physical exam shows choreiform movements. MRI shows caudate nucleus atrophy.
Which genetic concept best explains the earlier onset in the daughter compared with her father and grandfather?
A. Anticipation due to trinucleotide repeat expansion
B. Genomic imprinting
C. Variable penetrance
D. Locus heterogeneity
E. Allelic heterogeneity
Correct Answer: A. Anticipation due to trinucleotide repeat expansion
Why A Is Correct: Anticipation (Trinucleotide Repeat Expansion)
Anticipation = earlier age of onset and/or increased severity in successive generations, classically due to unstable repeat expansions.
This vignette strongly suggests Huntington disease (HD):
- AD inheritance
- Chorea + psychiatric changes + cognitive decline
- Caudate atrophy on imaging
- Caused by CAG repeat expansion in the HTT gene → polyglutamine (toxic gain-of-function)
High-yield facts (Step 1/2)
- HD: CAG (polyglutamine), AD, chromosome 4
- Anticipation often more pronounced with paternal transmission in HD (repeat instability during spermatogenesis)
- Bigger repeat number → typically earlier onset (esp. juvenile HD with large expansions)
Memory hook:
- CAG = glutamine
- PolyQ disease (Q = glutamine)
Systematically Destroying the Distractors
B. Genomic imprinting
Why it’s tempting: Parent-of-origin effects can change phenotype in children.
Why it’s wrong here:
Imprinting involves epigenetic silencing (e.g., DNA methylation) depending on whether the allele is inherited from the mother or father. It does not classically cause progressively earlier onset across generations.
Classic imprinting disorders:
- Prader-Willi syndrome: loss of paternal 15q11-q13 expression (hypotonia, hyperphagia, obesity, intellectual disability)
- Angelman syndrome: loss of maternal 15q11-q13 expression (ataxia, seizures, inappropriate laughter)
How to tell on exams: imprinting → same deletion region, different phenotype depending on parental origin.
C. Variable penetrance
Why it’s tempting: Families can show different expression patterns, and some individuals “skip” symptoms.
Why it’s wrong here:
Penetrance is the probability that a person with a genotype shows the phenotype (all-or-none concept at the population level). Variable/incomplete penetrance can make disease appear to skip generations, but it does not explain earlier onset with increasing severity across generations.
High-yield contrast:
- Penetrance: whether disease shows up
- Expressivity: how severe it is when it shows up
- Anticipation: severity/onset changes due to repeat expansion across generations
D. Locus heterogeneity
Why it’s tempting: Genetic disorders can have the “same phenotype” from different genes.
Why it’s wrong here:
Locus heterogeneity means mutations in different genes can cause similar clinical syndromes. It does not explain worsening/earlier onset across generations within one family.
High-yield examples:
- Retinitis pigmentosa (many different genes)
- Osteogenesis imperfecta-like phenotypes can have different genetic causes (broader concept)
- Some cardiomyopathies and deafness syndromes also demonstrate locus heterogeneity
Exam clue: heterogeneous loci → multiple genes, same phenotype.
E. Allelic heterogeneity
Why it’s tempting: Different mutations in the same gene can create different clinical pictures.
Why it’s wrong here:
Allelic heterogeneity means different mutations in the same gene can cause the same disorder (or varied severity), but it does not produce a generational pattern of earlier onset due to repeat expansion.
High-yield example:
- Cystic fibrosis (CFTR): many different alleles (e.g., ΔF508 and others) → CF phenotype
- β-thalassemia: many mutations in HBB → variable severity
Exam clue: “many different mutations in one gene cause the disorder.”
Anticipation: The Repeat Expansion Hall of Fame (USMLE Table)
| Disease | Repeat | Key clue | Inheritance/Pattern |
|---|---|---|---|
| Huntington disease | CAG | chorea, psych/cognitive decline, caudate atrophy | AD, often paternal anticipation |
| Myotonic dystrophy type 1 | CTG | myotonia, distal weakness, cataracts, cardiac conduction issues | AD, often maternal (congenital form) |
| Fragile X syndrome | CGG | intellectual disability, macroorchidism, long face, autism features | X-linked, maternal expansion risk |
| Friedreich ataxia | GAA | ataxia, hypertrophic cardiomyopathy, diabetes | AR |
High-yield rule of thumb:
Repeat expansions are dynamic mutations → unstable repeats expand in meiosis → phenotype can intensify over generations.
Rapid Exam Approach: How to Spot Anticipation in a Stem
Look for:
- Family history across generations
- Younger onset in child vs parent
- More severe phenotype in child vs parent
- A disorder you associate with repeat expansion (HD, myotonic dystrophy, Fragile X, Friedreich)
Then ask:
- Is there a known parent-of-origin bias?
- HD: often paternal
- Fragile X & congenital myotonic dystrophy: often maternal
Key Takeaways (High-Yield)
- Anticipation = earlier onset/more severe disease in successive generations, usually due to trinucleotide repeat expansion.
- In Huntington disease, CAG repeats expand and often show paternal anticipation.
- Imprinting = parent-of-origin gene expression differences (Prader-Willi vs Angelman).
- Penetrance/expressivity change who is affected and how they present, but do not create a repeat-driven generational worsening pattern.
- Locus vs allelic heterogeneity explain genetic diversity of similar phenotypes—not anticipation.