Molecular biology and genetics: Angelman syndrome: 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 · Molecular biology and genetics: Angelman syndrome

Sample item

A 3-year-old has severe developmental delay, absent speech, ataxic gait, seizures and frequent unprovoked laughter. FISH shows a deletion at 15q11-q13. On which parental chromosome is the deletion likely to be, and why does it matter?

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

The answer and the reasoning

D. The maternal chromosome, because the maternal UBE3A allele is the active one in neurons

In neurons the paternal UBE3A allele is silenced by imprinting, so only the maternal copy is expressed. Loss of maternal function, most often through a maternal 15q11-q13 deletion, causes Angelman syndrome. A paternal deletion of the same region causes Prader-Willi syndrome.

Option by option

  • A. The maternal chromosome, because the paternal allele is deleted in all cells

    The deletion is on the maternal chromosome, but the paternal allele is intact and silenced, not deleted.

  • B. The paternal chromosome, because the paternal UBE3A allele is the active one in neurons

    A paternal deletion of this region causes Prader-Willi syndrome, with hyperphagia and hypotonia.

  • C. Either chromosome, because imprinting does not affect this region

    The region is imprinted, so the parent of origin of the deletion determines the syndrome.

  • D. The maternal chromosome, because the maternal UBE3A allele is the active one in neurons (correct)

    Correct. Only the maternal UBE3A is active in neurons, so a maternal deletion causes Angelman syndrome.

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, The maternal chromosome, because the maternal UBE3A allele is the active one in neurons. In neurons the paternal UBE3A allele is silenced by imprinting, so only the maternal copy is expressed.

Why is "The maternal chromosome, because the paternal allele is deleted in al…" not the answer?

The deletion is on the maternal chromosome, but the paternal allele is intact and silenced, not deleted.

Why is "The paternal chromosome, because the paternal UBE3A allele is the act…" not the answer?

A paternal deletion of this region causes Prader-Willi syndrome, with hyperphagia and hypotonia.

Why is "Either chromosome, because imprinting does not affect this region" not the answer?

The region is imprinted, so the parent of origin of the deletion determines the syndrome.

What concept does this question test?

It tests Molecular biology and genetics: Angelman syndrome in Biochemistry. Loss of maternal function, most often through a maternal 15q11-q13 deletion, causes Angelman syndrome. A paternal deletion of the same region causes Prader-Willi syndrome.

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 Molecular biology and genetics: Angelman syndrome?

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.