Clinical GeneticsMarch 21, 20264 min read

Q-Bank Breakdown: Locus heterogeneity — Why Every Answer Choice Matters

Clinical vignette on Locus heterogeneity. Explain correct answer, then systematically address each distractor. Tag: Genetics > Clinical Genetics.

Q-Bank Breakdown: Locus heterogeneity — Why Every Answer Choice Matters

Tag: Genetics > Clinical Genetics

Clinical genetics questions love to test why two patients with the “same disease” don’t always share the same mutation. That concept is locus heterogeneity—and it’s a frequent source of tricky distractors on Step 1 and Step 2.

Clinical Vignette (Q-bank style)

A 6-year-old child is evaluated for chronic productive cough, recurrent sinopulmonary infections, and poor weight gain. Sweat chloride testing is elevated. Genetic testing for the most common mutation in the CFTR gene is negative. The parents ask how the child can have cystic fibrosis if the test was negative.

Which of the following best explains this finding?

A. Allelic heterogeneity
B. Anticipation
C. Locus heterogeneity
D. Variable expressivity
E. Incomplete penetrance

Correct Answer: A. Allelic heterogeneity

Why it’s correct

This stem screams cystic fibrosis (CF): recurrent respiratory infections + poor growth + elevated sweat chloride. The trick is that a “common mutation panel” can miss disease-causing variants.

  • Allelic heterogeneity = different mutations within the same gene cause the same disease/phenotype.
  • CF is classic: many different CFTR mutations can cause CF.
  • So a negative test for the most common CFTR mutation does not rule out CF; the patient may carry a different CFTR variant.

High-yield take-home:
If the diagnosis is clinically strong but “common mutation testing” is negative, think allelic heterogeneity (same locus/gene, different alleles).

What the question is really testing: Locus vs Allelic heterogeneity

Quick definitions you should be able to say in one breath

  • Allelic heterogeneity: same gene, different mutations → same disease
    • Example: CFTR mutations → cystic fibrosis; FBN1 mutations → Marfan syndrome
  • Locus heterogeneity: different genes (different loci) → same phenotype
    • Example: Osteogenesis imperfecta (usually COL1A1 or COL1A2), retinitis pigmentosa (many genes)

Systematic Distractor Breakdown (Why each wrong answer is wrong)

B. Anticipation — Wrong

Anticipation = disease becomes more severe and/or earlier onset in successive generations, classically due to trinucleotide repeat expansion.

  • Prototype disorders:
    • Huntington disease (CAG)
    • Myotonic dystrophy (CTG)
    • Fragile X syndrome (CGG)
    • Friedreich ataxia (GAA)

Why it doesn’t fit:
CF is not a trinucleotide repeat disorder, and the stem is about negative common mutation testing, not worsening across generations.

C. Locus heterogeneity — Tempting, but wrong for this stem

Locus heterogeneity means mutations in different genes produce the same phenotype.

Why it’s tempting:
The stem points to “genetic testing negative,” which can make you think “different gene.”

Why it’s wrong here:
CF is defined by pathogenic variants in CFTR (the disease is tied to this gene). A negative result on a common-mutation panel suggests:

  • the patient likely has a different CFTR mutation (allelic heterogeneity), or
  • the assay didn’t cover their variant (test limitations).

Step-ready nuance:
When a question specifically says “negative for the most common mutation,” it’s pushing you toward allelic heterogeneity (same gene, other mutations). Locus heterogeneity is more likely when the disease entity itself can be caused by different genes (e.g., retinitis pigmentosa).

D. Variable expressivity — Wrong

Variable expressivity = same genotype → different degrees or features of phenotype among individuals.

  • Example: NF1 can range from café-au-lait spots only to neurofibromas + optic gliomas.

Why it doesn’t fit:
The issue isn’t differing severity. The issue is a negative common mutation test despite a fitting phenotype.

E. Incomplete penetrance — Wrong

Incomplete penetrance = a person has the genotype but does not express the phenotype.

  • Example: some individuals with BRCA variants never develop cancer.
  • Classic association: “skips generations” in autosomal dominant conditions.

Why it doesn’t fit:
This child has the phenotype. The question is about why genetic testing might miss the mutation—not why someone lacks symptoms.

High-Yield Genetics Pearls (USMLE-friendly)

How to spot allelic heterogeneity in a vignette

Look for:

  • A classic phenotype of a known monogenic disease
  • Genetic testing that checked only common variants
  • Language like:
    • “negative for the most common mutation”
    • “targeted mutation panel was negative”
    • “screening test negative, but clinical suspicion remains high”

How to spot locus heterogeneity

Look for:

  • A phenotype that is known to arise from multiple genes
  • A question explicitly asking why different families or different patients can have the same disorder with mutations in different genes

Board-relevant examples to memorize

  • Allelic heterogeneity:
    • CF (many CFTR variants)
    • β-thalassemia (many HBB variants)
    • Marfan (many FBN1 variants)
  • Locus heterogeneity:
    • Osteogenesis imperfecta (COL1A1, COL1A2)
    • Retinitis pigmentosa (many different genes)
    • Hereditary deafness (common board-style example of multiple genes → similar phenotype)

Rapid-Fire Review (One-liners)

  • Allelic heterogeneity: different mutations, same gene → same disease
  • Locus heterogeneity: mutations in different genes → same phenotype
  • Anticipation: earlier onset/worse severity in later generations (repeat expansions)
  • Variable expressivity: same mutation → different severity/features
  • Incomplete penetrance: mutation present → phenotype absent in some carriers

Final Takeaway

When a vignette gives a textbook phenotype (like CF) but says “testing for the most common mutation is negative,” the best explanation is usually allelic heterogeneity, not locus heterogeneity. On USMLE, that single phrase often determines the entire question.

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