Genetics: trinucleotide repeat expansion: 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.

Try it

Anatomy · Genetics: trinucleotide repeat expansion

Sample item

A 3-year-old boy has developmental delay and a long face with large ears. His mother carries a premutation of about 90 CGG repeats in FMR1, and he has more than 200 repeats. During which process did the expansion to a full mutation occur?

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

The answer and the reasoning

D. Oogenesis in the premutation carrier mother

In fragile X syndrome a premutation (55 to 200 repeats) expands to a full mutation (more than 200 repeats, with methylation and silencing of FMR1) only when transmitted through the maternal germ line, and the risk rises with the number of repeats. Premutation fathers pass the premutation to daughters without expansion to a full mutation.

Option by option

  • A. Only after random X inactivation in the embryo

    X inactivation does not alter repeat number; methylation follows expansion.

  • B. Equally in oogenesis or spermatogenesis

    Expansion is not symmetrical between sexes, so the parent of origin matters.

  • C. Spermatogenesis in a premutation carrier father

    Premutations in sperm do not expand to full mutations; they usually shrink or stay stable.

  • D. Oogenesis in the premutation carrier mother (correct)

    Correct. Full-mutation expansion occurs in female meiosis, and larger premutations carry a greater risk.

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.

More Anatomy questions

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is D, Oogenesis in the premutation carrier mother. In fragile X syndrome a premutation (55 to 200 repeats) expands to a full mutation (more than 200 repeats, with methylation and silencing of FMR1) only when transmitted through the maternal germ line, and the risk rises with the number of repeats.

Why is "Only after random X inactivation in the embryo" not the answer?

X inactivation does not alter repeat number; methylation follows expansion.

Why is "Equally in oogenesis or spermatogenesis" not the answer?

Expansion is not symmetrical between sexes, so the parent of origin matters.

Why is "Spermatogenesis in a premutation carrier father" not the answer?

Premutations in sperm do not expand to full mutations; they usually shrink or stay stable.

What concept does this question test?

It tests Genetics: trinucleotide repeat expansion in Anatomy. Premutation fathers pass the premutation to daughters without expansion to a full mutation.

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 Genetics: trinucleotide repeat expansion?

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.