Q-Bank Breakdown: Mosaicism — Why Every Answer Choice Matters
System: Genetics
Topic: Clinical Genetics
Tag: Genetics > Clinical Genetics
Mosaicism is a classic “test-writer’s playground” because it sits at the intersection of embryology timing, chromosome biology, and clinical pattern recognition. In Q-banks, the right answer often hinges on when the mutation occurred and which tissues are involved—while distractors try to pull you toward inherited disorders, imprinting, or chimerism.
Clinical Vignette (USMLE Style)
A 2-year-old girl is evaluated for asymmetric body growth and skin findings. Pregnancy and delivery were uncomplicated. Physical exam shows hemihyperplasia (right leg larger than left) and streaks of hyperpigmented skin that follow the lines of Blaschko on the trunk. Development is normal. Family history is unremarkable. A peripheral blood karyotype is normal. A skin biopsy from a hyperpigmented streak reveals two cell lines: one with a normal karyotype and one with a chromosomal abnormality.
Which of the following best explains this patient’s findings?
A. Germline mutation inherited in an autosomal dominant pattern with variable expressivity
B. Postzygotic mitotic nondisjunction leading to mosaicism
C. Meiotic nondisjunction leading to complete trisomy in all cells
D. Robertsonian translocation transmitted from a balanced carrier parent
E. Maternal uniparental disomy due to trisomy rescue
F. Fusion of two zygotes leading to chimerism
Correct Answer: B. Postzygotic mitotic nondisjunction leading to mosaicism
Why this is correct
This child has segmental/asymmetric findings (hemihyperplasia, Blaschko-linear pigmentation), which strongly suggests mosaicism—two genetically distinct cell populations originating from one zygote due to a postzygotic event (after fertilization).
Key clues:
- Lines of Blaschko: reflect clonal expansion of genetically distinct skin cell populations during embryogenesis → classic for cutaneous mosaicism.
- Asymmetry (hemihyperplasia): suggests only some tissues are affected.
- Normal blood karyotype does not exclude mosaicism: the abnormal cell line may be confined to skin/other tissues, so testing the affected tissue is higher yield.
High-yield mosaicism facts (Step 1/2)
- Mosaicism = postzygotic mutation → ≥2 genetically different cell lines from one zygote.
- Earlier postzygotic error → more tissues involved and often more severe phenotype.
- Later postzygotic error → localized/segmental disease (often skin findings).
- Mosaic disorders often show:
- Patchy skin pigmentation
- Asymmetric growth
- Variable organ involvement based on tissue distribution
Why Every Distractor Is Wrong (and When It Would Be Right)
A. Germline mutation inherited in an autosomal dominant pattern with variable expressivity
Why it’s wrong here:
An inherited germline mutation is present in every cell, so it typically produces systemic and often bilateral/symmetric findings (though severity can vary). It would not classically produce Blaschko-linear streaks due to cell-line patchwork.
When it could be right:
- Classic AD conditions with variable expressivity (e.g., Neurofibromatosis type 1) where family history may be present (though de novo mutations also occur).
- But NF1 lesions (café-au-lait, neurofibromas) are not described as strictly Blaschko-linear in the typical presentation.
USMLE pearl:
- Variable expressivity = same genotype, different severity
- Incomplete penetrance = genotype may show no phenotype
C. Meiotic nondisjunction leading to complete trisomy in all cells
Why it’s wrong here:
Meiotic nondisjunction happens before fertilization (during gametogenesis), producing a zygote with the aneuploidy in all cells. That typically causes diffuse findings and a consistent abnormality across tissues (blood karyotype would usually show it).
When it could be right:
- Down syndrome (Trisomy 21) due to meiotic nondisjunction: hypotonia, characteristic facies, congenital heart disease, etc.
- These are not patchy/segmental distributions.
USMLE pearl:
- Mosaic trisomy can happen via postzygotic mitotic nondisjunction or via trisomy rescue—but complete trisomy in all cells points to meiosis, not mosaic patterns.
D. Robertsonian translocation transmitted from a balanced carrier parent
Why it’s wrong here:
Robertsonian translocations are structural rearrangements (fusion of acrocentric chromosomes, usually 13, 14, 15, 21, 22). They cause problems in offspring due to unbalanced segregation but do not create patchwork cell populations with Blaschko lines.
When it could be right:
- Familial Down syndrome (most commonly t(14;21)): patient has trisomy 21 phenotype; parent is a balanced carrier.
USMLE pearl:
- Robertsonian translocation: 45 chromosomes in balanced carrier, phenotypically normal
- Offspring can have translocation trisomy 21
E. Maternal uniparental disomy due to trisomy rescue
Why it’s wrong here:
Trisomy rescue can create uniparental disomy (UPD) and sometimes mosaicism, but the vignette emphasizes two distinct karyotypes in affected skin, and classic Blaschko + asymmetry most strongly supports a postzygotic mitotic error producing mosaicism.
Also, UPD is more about imprinting disorders rather than linear skin findings.
When it could be right:
- Prader-Willi syndrome: paternal deletion of 15q or maternal UPD(15)
- Angelman syndrome: maternal deletion of 15q or paternal UPD(15)
- Beckwith-Wiedemann syndrome: imprinting abnormalities on chromosome 11p15 (can be associated with hemihyperplasia and tumor risk; mosaic imprinting defects can occur)
USMLE pearl:
- UPD = both chromosomes from one parent → imprinting effects
- Trisomy rescue can yield mosaicism, but clueing is often through imprinting phenotypes or placental mosaicism
F. Fusion of two zygotes leading to chimerism
Why it’s wrong here:
Chimerism involves two genetically distinct zygotes fusing into one individual → two cell lines from two different embryos. It can present with patchy pigmentation or ambiguous genitalia, but it’s rarer and often associated with:
- Mixed blood types
- Two different DNA profiles in different tissues
- Sometimes “tetragametic” findings
The vignette points to a postzygotic event (mitotic error) within one embryo rather than fusion of two embryos.
When it could be right:
- Discrepant genetic testing results (e.g., different STR profiles between blood and saliva)
- Chimerism after bone marrow transplant (acquired chimera)
USMLE pearl:
- Mosaic: one zygote, postzygotic mutation
- Chimera: two zygotes (or acquired via transplant)
The Exam Strategy: How to Spot Mosaicism Fast
Look for:
- Asymmetry (hemihyperplasia, unilateral findings)
- Patchy/linear skin lesions (esp. lines of Blaschko)
- Normal genetic test in blood but abnormal results in affected tissue
- “Two cell lines” on biopsy or cytogenetics
Testing tip (Step 2–style): If mosaicism is suspected and blood testing is negative, the next step is often genetic testing of the affected tissue (skin biopsy, tumor tissue, etc.).
High-Yield Summary Table
| Concept | Key Feature | Classic Clue |
|---|---|---|
| Mosaicism | Postzygotic mutation in one zygote | Blaschko lines, asymmetry, tissue-limited abnormalities |
| Meiotic nondisjunction | Aneuploidy in all cells | Abnormal blood karyotype; diffuse phenotype |
| Robertsonian translocation | Structural rearrangement of acrocentric chromosomes | Familial Down syndrome; balanced carrier parent |
| UPD (trisomy rescue) | Both homologs from one parent | Imprinting disorders (PWS/Angelman, BWS mechanisms) |
| Chimerism | Two zygotes or transplant | Mixed blood type/DNA profiles, rare congenital presentations |
Take-Home Points (What USMLE Wants You to Say)
- Mosaicism = postzygotic mitotic error → two cell lines in one individual.
- Lines of Blaschko + asymmetry are high-yield clinical tells.
- A normal blood karyotype does not rule out mosaicism—test the affected tissue.
- Distinguish mosaicism from chimerism (two zygotes) and from meiotic nondisjunction (all cells affected).