Genetics: dystrophin gene mutations: practice question

A Anatomy 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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Anatomy · Genetics: dystrophin gene mutations

Sample item

Two boys have deletions in the dystrophin gene. Boy A loses ambulation at 11 years, and boy B walks until his thirties with milder weakness. Which statement best explains the difference?

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

The answer and the reasoning

D. Boy A has a frameshift deletion, and boy B has an in-frame deletion giving a shortened, partly functional protein

The reading-frame rule explains Duchenne and Becker muscular dystrophy, both caused by mutations in the same X-linked gene at Xp21. Out-of-frame deletions truncate dystrophin and give almost no functional protein (Duchenne), whereas in-frame deletions keep the ends of the protein and give a reduced or altered but partly functional one (Becker).

Option by option

  • A. Boy A has maternal X-inactivation, and boy B has paternal X-inactivation

    X-inactivation matters for female carriers and does not change disease severity in hemizygous boys.

  • B. Boy A has a mitochondrial mutation, and boy B has a nuclear mutation

    Both conditions are caused by nuclear X-linked mutations, not mitochondrial ones.

  • C. Boy A has a trinucleotide repeat expansion, and boy B has a point mutation in an autosomal gene

    Both diseases arise from dystrophin gene mutations on the X chromosome, not repeat expansions or an autosomal gene.

  • D. Boy A has a frameshift deletion, and boy B has an in-frame deletion giving a shortened, partly functional protein (correct)

    Correct. The reading frame decides whether any functional dystrophin is made.

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 Anatomy question

What is the answer to this Anatomy question?

The best answer is D, Boy A has a frameshift deletion, and boy B has an in-frame deletion giving a shortened, partly functional protein. The reading-frame rule explains Duchenne and Becker muscular dystrophy, both caused by mutations in the same X-linked gene at Xp21.

Why is "Boy A has maternal X-inactivation, and boy B has paternal X-inactivat…" not the answer?

X-inactivation matters for female carriers and does not change disease severity in hemizygous boys.

Why is "Boy A has a mitochondrial mutation, and boy B has a nuclear mutation" not the answer?

Both conditions are caused by nuclear X-linked mutations, not mitochondrial ones.

Why is "Boy A has a trinucleotide repeat expansion, and boy B has a point mut…" not the answer?

Both diseases arise from dystrophin gene mutations on the X chromosome, not repeat expansions or an autosomal gene.

What concept does this question test?

It tests Genetics: dystrophin gene mutations in Anatomy. Out-of-frame deletions truncate dystrophin and give almost no functional protein (Duchenne), whereas in-frame deletions keep the ends of the protein and give a reduced or altered but partly functional one (Becker).

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Genetics: dystrophin gene mutations?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy 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.