Mitochondrial inheritance: Leber optic neuropathy: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Mitochondrial inheritance: Leber optic neuropathy

Sample item

A 22-year-old man develops painless, sequential loss of central vision over several weeks. His maternal uncle and maternal grandmother had similar loss, but his father's relatives are healthy. A mutation in a mitochondrial gene for a complex I subunit is found. What is the expected inheritance of this disease?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Maternal transmission to children of both sexes, and an affected father does not transmit it

Mitochondrial DNA is inherited from the oocyte, so a mother transmits her mtDNA to all of her children but a father transmits none. Leber hereditary optic neuropathy is a classic example, with a higher penetrance in males. Heteroplasmy and threshold effects account for the variable expression within a family.

Option by option

  • A. X-linked recessive, with carrier mothers and affected sons only

    An X-linked recessive disease would not be attributed to a mitochondrial gene mutation and affects daughters rarely.

  • B. Autosomal recessive, with both parents carriers

    A recessive disease would not follow a clear maternal line in a multi-generation pedigree.

  • C. Autosomal dominant, with male-to-male transmission

    Male-to-male transmission excludes mitochondrial inheritance.

  • D. Maternal transmission to children of both sexes, and an affected father does not transmit it (correct)

    Mitochondrial DNA passes down the maternal line to sons and daughters.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Maternal transmission to children of both sexes, and an affected father does not transmit it. Mitochondrial DNA is inherited from the oocyte, so a mother transmits her mtDNA to all of her children but a father transmits none.

Why is "X-linked recessive, with carrier mothers and affected sons only" not the answer?

An X-linked recessive disease would not be attributed to a mitochondrial gene mutation and affects daughters rarely.

Why is "Autosomal recessive, with both parents carriers" not the answer?

A recessive disease would not follow a clear maternal line in a multi-generation pedigree.

Why is "Autosomal dominant, with male-to-male transmission" not the answer?

Male-to-male transmission excludes mitochondrial inheritance.

What concept does this question test?

It tests Mitochondrial inheritance: Leber optic neuropathy in Biochemistry. Leber hereditary optic neuropathy is a classic example, with a higher penetrance in males. Heteroplasmy and threshold effects account for the variable expression within a family.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Mitochondrial inheritance: Leber optic neuropathy?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.