USMLE Step 1 · Biochemistry & Genetics

X-linked recessive inheritance vs X-linked dominant inheritance

How to tell X-linked recessive inheritance and X-linked dominant inheritance apart when a board question makes them look alike.

Fastest separator

X-linked recessive is mostly males through carrier mothers; X-linked dominant makes all daughters of affected fathers affected and may look female-only when male-lethal.

Side by side

X-linked recessive inheritanceX-linked dominant inheritance

Key clue

X-linked recessive inheritance mainly affects males, is transmitted through carrier mothers, has no father-to-son transmission, and affected fathers make all daughters carriers.

X-linked dominant inheritance affects both sexes, often more females, has no father-to-son transmission, affected fathers transmit disease to all daughters, and male-lethal disorders can appear female-only.

Mechanism

A hemizygous male expresses a pathogenic X-linked allele inherited from his mother.

One pathogenic X-linked allele can cause disease in females or males, with male disease sometimes more severe or lethal.

Labs and tests

Affected males connected through maternal line and skipped generations support X-linked recessive disease.

Affected father with all affected daughters and no affected sons supports X-linked dominant disease.

The trap

Both lack father-to-son transmission, so daughter phenotype after an affected father is the key split.

Watch it explained

60-second lessonX-Linked Recessive vs Dominant: Reading the Pedigree (USMLE Step 1)

Quiz yourselfX-Linked Recessive or Dominant? 6-Second Genetics Pop Quiz (USMLE Step 1)

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The repair move

For X-linked pedigrees, first rule out father-to-son transmission, then inspect daughters of affected fathers.

Try a free question

Core

A pedigree shows hemophilia A across four generations. All affected individuals are male. They are linked through carrier mothers (unaffected daughters of affected grandfathers). NO father-to-son transmission is seen. Daughters of affected fathers are obligate carriers but unaffected. Which pattern of inheritance is most likely?

Show answer and explanation

Answer: A. X-linked recessive. X-linked recessive inheritance affects predominantly males through carrier mothers, has NO father-to-son transmission (sons inherit the Y), and daughters of affected fathers are obligate carriers. Hemophilia A/B, Duchenne muscular dystrophy, G6PD deficiency, Lesch-Nyhan, Wiskott-Aldrich, X-SCID are classic examples.

Why the other choices are tempting

B. X-linked dominant would have affected daughters of affected fathers, not just unaffected carriers.
C. Autosomal recessive would not show such strong male predominance.
D. Autosomal dominant would show father-to-son transmission and equal sex involvement.
E. Mitochondrial inheritance would have all children of an affected mother at risk, but here only the carrier daughter status is the carrier line.

Flashcards

Tap a card to flip it.

What clue supports X-linked recessive inheritance?

Choose X-linked recessive inheritance when the stem gives this pattern: X-linked recessive inheritance mainly affects males, is transmitted through carrier mothers, has no father-to-son transmission, and affected fathers make all daughters carriers.

What clue supports X-linked dominant inheritance?

Choose X-linked dominant inheritance when the stem gives this contrasting pattern: X-linked dominant inheritance affects both sexes, often more females, has no father-to-son transmission, and affected fathers transmit disease to all daughters.

Quick answers

What is the fastest way to tell X-linked recessive inheritance from X-linked dominant inheritance?