Clinical GeneticsMarch 21, 20264 min read

Q-Bank Breakdown: Linkage disequilibrium — Why Every Answer Choice Matters

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

Q-Bank Breakdown: Linkage disequilibrium — Why Every Answer Choice Matters

Tag: Genetics > Clinical Genetics

Linkage disequilibrium (LD) is one of those genetics concepts that shows up in vignettes disguised as population risk, ancestry, “founder mutations,” or GWAS-style association data. The key to getting these questions right is knowing what LD is, what it is not, and why the wrong answer choices are tempting.

Clinical Vignette (USMLE-Style)

A 29-year-old woman of Northern European ancestry is found to carry the HLA-B27 allele. Her physician explains that this marker is associated with a higher risk of ankylosing spondylitis. She asks why a gene involved in antigen presentation would predict an inflammatory spine disease. The physician notes that a nearby genetic variant increases susceptibility and tends to be inherited together with HLA-B27 more often than would be expected by chance.

Which concept best explains this association?

A. Antigenic drift
B. Linkage disequilibrium
C. Hardy–Weinberg equilibrium
D. Codominance
E. Variable expressivity

Correct Answer: B. Linkage disequilibrium

What LD means

Linkage disequilibrium is the non-random association of alleles at different loci in a population. In plain terms:

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Two genetic variants are found together on the same chromosome more often than expected based on their individual frequencies.

This is commonly due to:

  • Physical proximity on the chromosome → low recombination between loci
  • Founder effect, population bottlenecks, and population stratification
  • Recent positive selection (haplotypes “hitchhike” together)

Why HLA associations scream LD

Many classic disease-marker associations (especially in the HLA region on chromosome 6) exist because:

  • The region is gene-dense
  • There are characteristic haplotypes
  • Recombination patterns can preserve blocks of linked variants

So HLA-B27 can function as a marker for nearby causal variant(s)—even if HLA-B27 itself is not the direct cause.

High-Yield LD Facts (Step 1 + Step 2)

  • LD ≠ linkage.
    • Linkage = loci close together tend to be inherited together within families (pedigree concept).
    • LD = alleles co-occur more than expected in a population (population genetics concept).
  • LD enables GWAS. SNPs on genotyping chips can “tag” nearby causal variants because of LD blocks.
  • LD decays over generations via recombination; thus, LD is often stronger when:
    • a mutation is recent
    • a population experienced a bottleneck/founder effect
  • Common USMLE phrasing: “marker is associated with disease because it’s near the causative gene and inherited together.”

Why Every Answer Choice Matters (Systematic Distractor Breakdown)

A. Antigenic drift — Wrong

Antigenic drift is:

  • Gradual accumulation of mutations in viral genomes (especially influenza) due to error-prone replication
  • Leads to seasonal variation and the need for updated vaccines

Why it’s tempting: the vignette mentions HLA/antigen presentation.
Why it’s wrong: LD is about human population allele co-inheritance, not viral evolution.

USMLE tie-in:

  • Drift = minor changes (point mutations) → influenza A/B seasonal changes
  • Shift = major reassortment (segmented genome) → pandemics (influenza A)

C. Hardy–Weinberg equilibrium — Wrong

Hardy–Weinberg describes expected genotype frequencies in an ideal population:

  • No selection, mutation, migration
  • Random mating
  • Large population (no genetic drift)

It answers questions like:

  • “If the carrier frequency is X, what is disease prevalence?”

Why it’s tempting: both are population genetics.
Why it’s wrong: Hardy–Weinberg predicts frequencies assuming random association; LD is specifically about non-random association between loci.

High yield:
Hardy–Weinberg = one locus genotype math (e.g., p2+2pq+q2=1p^2 + 2pq + q^2 = 1)
LD = relationships between loci (haplotypes, “tag SNPs”).

D. Codominance — Wrong

Codominance means:

  • Both alleles are fully expressed in heterozygotes
    Classic example: AB blood type (IAIB)

Why it’s tempting: HLA genes are often highly polymorphic and immunologically relevant.
Why it’s wrong: codominance is about expression pattern, not about co-inheritance patterns across loci.

USMLE link:

  • Codominance: AB blood type, HLA allele expression
  • Incomplete dominance: intermediate phenotype (e.g., familial hypercholesterolemia heterozygote severity)

E. Variable expressivity — Wrong

Variable expressivity:

  • Same genotype → different severity or features among individuals

Examples:

  • Neurofibromatosis type 1: café-au-lait spots ± neurofibromas ± optic gliomas
  • Tuberous sclerosis: varying manifestations

Why it’s tempting: many genetic conditions have variable clinical presentations.
Why it’s wrong: the question asks why a marker allele predicts disease risk—that’s LD/association, not phenotype variability.

High yield distinction:

  • Incomplete penetrance: some with genotype show no phenotype
  • Variable expressivity: all show phenotype, but severity/features differ

How to Spot LD in a Vignette (Fast Pattern Recognition)

Look for phrases like:

  • “Associated with” (without claiming causation)
  • “Marker for disease risk”
  • “Inherited together more often than expected”
  • “Nearby locus”
  • “Haplotype”
  • “Founder mutation common in a population”
  • “GWAS found SNP associated with disease”

Then pick linkage disequilibrium.

Board-Style Takeaway

Linkage disequilibrium explains why an allele can be a strong risk marker even if it is not causal: it’s co-inherited with a nearby causal variant due to non-random association of alleles in a population.

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