Medical genetics: origin of nondisjunction: 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 · Medical genetics: origin of nondisjunction

Sample item

In most cases of trisomy 21 with a standard 47,XX,+21 karyotype, in which parent and at which stage did the error occur?

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

The answer and the reasoning

B. Maternal meiosis I

About nine in ten cases of standard trisomy 21 arise from nondisjunction in the mother, most often at meiosis I. The risk rises with maternal age because oocytes remain arrested in prophase I for decades.

Option by option

  • A. Maternal meiosis II

    Maternal meiosis II errors occur but are less common than meiosis I errors.

  • B. Maternal meiosis I (correct)

    Correct. Most extra chromosomes 21 come from the mother, usually from a meiosis I error.

  • C. Paternal meiosis I

    Paternal errors account for a small minority of cases.

  • D. Post-zygotic mitosis

    Post-zygotic mitotic errors produce mosaicism rather than the standard full trisomy.

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 B, Maternal meiosis I. About nine in ten cases of standard trisomy 21 arise from nondisjunction in the mother, most often at meiosis I.

Why is "Maternal meiosis II" not the answer?

Maternal meiosis II errors occur but are less common than meiosis I errors.

Why is "Paternal meiosis I" not the answer?

Paternal errors account for a small minority of cases.

Why is "Post-zygotic mitosis" not the answer?

Post-zygotic mitotic errors produce mosaicism rather than the standard full trisomy.

What concept does this question test?

It tests Medical genetics: origin of nondisjunction in Anatomy. The risk rises with maternal age because oocytes remain arrested in prophase I for decades.

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 Medical genetics: origin of nondisjunction?

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.