Everything You Need to Know About X-linked Disorders for Step 1

X-linked disorders are a Step 1 classic because they combine inheritance pattern recognition, molecular genetics, and high-yield clinical vignettes. If you can quickly identify the pedigree pattern and match hallmark features (e.g., “toddler boy + recurrent infections + no CD40L”), you’ll pick up fast points on exam day.

---

Where This Fits in Genetics (Step 1 Framing)

Definition (What “X-linked” Means)

An X-linked disorder is caused by a pathogenic variant in a gene located on the X chromosome.

Why It Matters (Key Biology)

• Males (46,XY) are hemizygous for X-linked genes → a single pathogenic variant on the X chromosome is enough to cause disease.
• Females (46,XX) may be:
  • Carriers (often asymptomatic in X-linked recessive disorders)
  • Affected (more likely in X-linked dominant disorders or due to skewed X-inactivation)

Classic USMLE Rule of Thumb

• X-linked recessive (XLR): “More males affected; females are often carriers.”
• X-linked dominant (XLD): “No male-to-male transmission; often more severe in males.”

First Aid cross-reference: Genetics—Modes of inheritance; Pedigrees; X-inactivation (Lyonization).

---

Pathophysiology: The “Why” Behind X-linked Patterns

X-inactivation (Lyonization)

In females, one X chromosome is randomly inactivated early in embryogenesis → forms a Barr body.

• Mosaicism: females can have mixed populations of cells expressing either the normal or mutant allele.
• Skewed X-inactivation can make carrier females symptomatic (HY Step 1 nuance).

First Aid cross-reference: Barr bodies; X-inactivation; mosaicism.

“No Male-to-Male Transmission” (Must-Know)

Fathers pass their Y chromosome to sons, not their X.

• If you see father → son transmission, it is not X-linked (think autosomal or Y-linked).

---

How to Recognize X-linked Inheritance on Pedigrees (Exam-Style)

X-linked Recessive (XLR): High-yield pedigree clues

• Mostly males affected
• Affected males often born to carrier mothers
• Skips generations via carrier females
• All daughters of an affected male are carriers (if mother is unaffected)
• No father-to-son transmission

Probability checkpoints

• Carrier mother (XᴺXᵐ) + normal father (XᴺY):
  • Sons: 50% affected
  • Daughters: 50% carriers

X-linked Dominant (XLD): High-yield pedigree clues

• Both sexes can be affected
• Often more females affected (because male disease can be severe/lethal)
• Affected father:
  • All daughters affected
  • No sons affected
• Affected mother:
  • 50% of sons and 50% of daughters affected (if heterozygous)

---

The Big X-linked Disorders You Need for Step 1 (Deep Dive)

Below are the “can’t-miss” X-linked disorders with path, presentation, diagnosis, and treatment—plus classic associations.

---

X-linked Recessive Disorders (Most Common on Step 1)

1) Duchenne Muscular Dystrophy (DMD)

Gene/Pathophysiology

• Mutation (often deletion) in dystrophin → unstable muscle membrane (sarcolemma) → muscle fiber degeneration
• Frameshift mutation classically → absent dystrophin (more severe)

Clinical presentation (HY)

• Boy with proximal muscle weakness
• Gowers sign
• Pseudohypertrophy of calves
• Progressive cardiomyopathy → dilated cardiomyopathy
• Often diagnosed in early childhood

Diagnosis

• ↑ CK
• Genetic testing for dystrophin mutation
• Muscle biopsy: absent dystrophin on immunostaining (if tested)

Treatment (boards-level)

• Glucocorticoids can slow progression
• Cardiac/respiratory support; PT
• Emerging mutation-specific therapies exist (not typically tested in detail)

First Aid cross-reference: Muscular dystrophies; Dystrophin; Gowers; Pseudohypertrophy.

Contrast to Becker

• Becker = non-frameshift → reduced/abnormal dystrophin → later onset, milder.

---

2) Hemophilia A and B

Gene/Pathophysiology

• Hemophilia A: Factor VIII deficiency
• Hemophilia B: Factor IX deficiency
  → impaired intrinsic pathway → decreased thrombin generation

Clinical presentation (HY)

• Hemarthroses
• Deep tissue bleeding, easy bruising
• Bleeding after procedures/circumcision

Labs

• ↑ PTT
• Normal PT
• Normal bleeding time, normal platelets
• Mixing study corrects PTT (suggests factor deficiency, not inhibitor)

Diagnosis

• Factor activity assay (VIII or IX)

Treatment

• Factor replacement (VIII or IX)
• Desmopressin (DDAVP) for mild hemophilia A (↑ vWF → stabilizes factor VIII)

First Aid cross-reference: Coagulation cascade; Hemophilias; DDAVP.

---

3) G6PD Deficiency

Pathophysiology

• ↓ G6PD → ↓ NADPH → ↓ reduced glutathione
  → RBCs vulnerable to oxidative stress → hemolysis

Triggers (HY mnemonic-style list)

• Fava beans
• Infections
• Drugs: sulfonamides, dapsone, primaquine, nitrofurantoin (plus others)

Clinical presentation

• Episodic jaundice, dark urine, pallor after trigger

Peripheral smear

• Heinz bodies (denatured Hb) on supravital stain
• Bite cells (splenic macrophages remove Heinz bodies)

Diagnosis

• Enzyme activity assay (note: can be falsely normal during acute hemolysis due to young RBCs)

Treatment

• Avoid triggers; supportive care during hemolytic episodes

First Aid cross-reference: Hemolytic anemias; Heinz bodies; Bite cells; Oxidative stress.

---

4) Bruton Agammaglobulinemia (X-linked Agammaglobulinemia)

Pathophysiology

• BTK mutation → failure of B cell maturation
  → ↓ B cells, ↓ all immunoglobulins

Clinical presentation (HY)

• Male infant with recurrent bacterial/enteroviral infections after 6 months (maternal IgG wanes)
• Absent tonsils/lymph nodes (lack of germinal centers)

Diagnosis

• ↓ CD19+ B cells
• Very low immunoglobulins

Treatment

• IVIG; avoid live vaccines

First Aid cross-reference: Primary immunodeficiencies; BTK; Absent germinal centers.

---

5) Wiskott–Aldrich Syndrome

Pathophysiology

• WAS gene defect → impaired actin cytoskeleton rearrangement in immune cells
  → combined immunodeficiency + platelet problem

Clinical triad (must know)

• Eczema
• Thrombocytopenia (classically small platelets)
• Recurrent infections

Diagnosis

• Low platelets + immune workup (often ↓ IgM, ↑ IgA/IgE)

Treatment

• Supportive care; HSCT can be curative

First Aid cross-reference: Immunodeficiencies; Thrombocytopenia with small platelets; Eczema.

---

6) Chronic Granulomatous Disease (CGD)

Pathophysiology

• Defective NADPH oxidase (often X-linked) → ↓ respiratory burst in phagocytes
  → impaired killing of catalase-positive organisms

Clinical presentation

• Recurrent infections, pneumonia/abscesses
• Catalase-positive organisms (HY list):
  • S. aureus, Burkholderia cepacia, Serratia, Nocardia, Aspergillus

Diagnosis

• Abnormal oxidative burst test:
  • DHR flow cytometry: decreased fluorescence
  • Nitroblue tetrazolium (NBT) stays negative

Treatment

• TMP-SMX prophylaxis, itraconazole, interferon-γ (classically taught)

First Aid cross-reference: Phagocyte disorders; Catalase-positive organisms; DHR/NBT.

---

7) X-linked Hyper-IgM Syndrome

Pathophysiology

• Defect in CD40L on Th cells → can’t class-switch B cells
  → high IgM, low IgG/IgA/IgE; no germinal centers

Clinical presentation

• Severe pyogenic infections, opportunistic infections (e.g., Pneumocystis)
• May present in infancy

Diagnosis

• ↑ IgM, ↓ others; absent class switching

Treatment

• IVIG; prophylaxis for opportunistic infections; HSCT

First Aid cross-reference: Class switching; CD40–CD40L; Hyper-IgM.

---

8) Lesch–Nyhan Syndrome

Pathophysiology

• HGPRT deficiency → impaired purine salvage
  → ↑ uric acid, neurobehavioral manifestations

Clinical presentation (HY)

• Self-injury (lip/finger biting)
• Dystonia/choreoathetosis
• Gout/kidney stones
• “Orange sand” crystals in diaper (urate)

Diagnosis

• Elevated uric acid; enzyme/genetic testing

Treatment

• Allopurinol/febuxostat for hyperuricemia (does not fix neurobehavioral symptoms)

First Aid cross-reference: Purine metabolism; HGPRT; Hyperuricemia.

---

9) Ornithine Transcarbamylase (OTC) Deficiency

Pathophysiology

• Urea cycle disorder → can’t clear ammonia effectively
  → hyperammonemia, ↑ orotic acid

Clinical presentation

• Vomiting, lethargy, cerebral edema; can be catastrophic in newborn males

Key lab association (HY)

• ↑ ammonia
• ↑ orotic acid
• ↓ BUN
• Normal UMP synthase (this differentiates from hereditary orotic aciduria)

Treatment

• Limit protein; nitrogen scavengers (benzoate/phenylbutyrate); acute management of hyperammonemia

First Aid cross-reference: Urea cycle; Hyperammonemia; Orotic acid.

---

10) Fabry Disease

Pathophysiology

• α-galactosidase A deficiency → globotriaosylceramide (ceramide trihexoside) accumulation

Clinical presentation (HY)

• Episodic peripheral neuropathic pain (acroparesthesias)
• Angiokeratomas
• Hypohidrosis
• Progressive renal failure, cardiac disease

Diagnosis

• Enzyme assay or genetic testing

Treatment

• Enzyme replacement therapy (ERT)

First Aid cross-reference: Lysosomal storage diseases; Angiokeratomas; Acroparesthesias.

---

X-linked Dominant Disorders (High-Yield “Pattern” Questions)

1) Fragile X Syndrome

Genetics/Pathophysiology

• CGG trinucleotide repeat expansion in FMR1
• Anticipation (worsens in successive generations)
• Often associated with maternal transmission of premutation → full mutation risk increases with repeat size

Clinical presentation (HY)

• Intellectual disability
• Autism features
• Long face, large ears
• Macroorchidism (post-pubertal)

Diagnosis

• PCR/Southern blot for CGG repeats and methylation status

First Aid cross-reference: Trinucleotide repeat disorders; Anticipation; Fragile X.

---

2) Rett Syndrome

Genetics/Pathophysiology

• MECP2 mutation (X-linked dominant), typically affects girls (often lethal in males)

Clinical presentation

• Normal early development then regression
• Loss of purposeful hand movements → hand-wringing
• Seizures, intellectual disability

First Aid cross-reference: Neurodevelopmental disorders; Rett.

---

3) Hypophosphatemic Rickets (X-linked Dominant)

Pathophysiology

• PHEX mutation → ↑ FGF23 → renal phosphate wasting
  → low phosphate → impaired bone mineralization

Clinical presentation

• Rickets/osteomalacia, bone pain, deformities
• Labs: low phosphate, normal or low vitamin D, variable PTH

Treatment

• Phosphate supplementation + active vitamin D (calcitriol)

First Aid cross-reference: Rickets; Phosphate regulation; FGF23.

---

Diagnosis Strategy: Step 1 Approach to X-linked Vignettes

1) Start with the “patient type”

• Young boy with severe disease? Think XLR.
• Father affected + all daughters affected? XLD.

2) Use the “signature clue”

• Gowers + calf pseudohypertrophy → DMD
• Hemarthroses + ↑PTT → Hemophilia
• Bite cells after sulfa → G6PD deficiency
• No B cells + no tonsils → Bruton
• Eczema + thrombocytopenia → Wiskott-Aldrich
• Catalase+ infections + abnormal DHR → CGD
• High IgM → Hyper-IgM (CD40L)
• Self-harm + hyperuricemia → Lesch–Nyhan
• Hyperammonemia + orotic acid → OTC
• Angiokeratomas + acroparesthesias → Fabry
• Macroorchidism + long face → Fragile X
• Hand-wringing regression in girl → Rett

---

Treatment Principles (What Step 1 Expects)

Step 1 usually focuses on recognizing the disorder and knowing first-line, mechanism-based therapy:

• Replace missing factors/enzymes: factor VIII/IX, ERT (Fabry)
• Avoid triggers: G6PD oxidant drugs/foods
• Immunologic support: IVIG (Bruton, Hyper-IgM), antimicrobial prophylaxis (CGD)
• Metabolic detox: nitrogen scavengers for urea cycle disorders
• Supportive/organ protection: steroids and cardiopulmonary care in DMD

---

High-Yield Associations & Rapid Review Table

---

First Aid Cross-References (Quick Navigation)

Look up these sections in First Aid for the USMLE Step 1:

• Modes of inheritance & pedigrees (XLR vs XLD, no male-to-male transmission)
• X-inactivation/Barr bodies
• Trinucleotide repeat disorders (Fragile X)
• Immunodeficiencies (Bruton, Wiskott-Aldrich, CGD, Hyper-IgM)
• Biochemistry:
  • Urea cycle (OTC)
  • Purine salvage (Lesch–Nyhan)
  • Lysosomal storage diseases (Fabry)
• Heme/Onc:
  • Coagulation (Hemophilia)
  • Hemolytic anemias (G6PD)
• MSK (Duchenne/Becker)

---

Final Step 1 Takeaways (Ultra High-Yield)

• No male-to-male transmission = think X-linked.
• XLR: males affected, females often carriers; can skip generations.
• XLD: affected father → all daughters affected, no sons affected.
• Don’t forget carrier females can be symptomatic due to skewed X-inactivation.
• Master the “signature clues” (DMD Gowers, CGD catalase+, Bruton's no B cells, Hyper-IgM class-switch defect, etc.).