Genetics: Robertsonian translocation: 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: Robertsonian translocation

Sample item

A phenotypically normal woman has had two children with Down syndrome and a miscarriage, and her husband has a normal karyotype. Her affected child's karyotype is 46,XY,der(14;21)(q10;q10),+21. What does her own karyotype most likely show?

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

The answer and the reasoning

D. 45 chromosomes with a balanced 14;21 Robertsonian translocation

A carrier of a balanced Robertsonian translocation has only 45 chromosomes, with two long arms fused, and is phenotypically normal. Her gametes can contain an extra copy of chromosome 21 material, giving translocation Down syndrome that recurs regardless of maternal age. Genetic counselling and parental karyotyping are indicated.

Option by option

  • A. 47 chromosomes with mosaic trisomy 21

    A mosaic trisomy 21 mother would usually show features of Down syndrome and would not explain the translocation chromosome.

  • B. 46 chromosomes with a balanced reciprocal translocation between 14 and 21

    A reciprocal translocation preserves 46 chromosomes and cannot produce a der(14;21) with fused long arms.

  • C. 46 chromosomes with a normal karyotype and general-population recurrence risk

    Normal parental karyotypes would imply an age-related low recurrence, which does not fit the translocation seen in the child.

  • D. 45 chromosomes with a balanced 14;21 Robertsonian translocation (correct)

    Correct. A balanced 14;21 carrier has 45 chromosomes, and recurrence risk is substantially raised.

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, 45 chromosomes with a balanced 14;21 Robertsonian translocation. A carrier of a balanced Robertsonian translocation has only 45 chromosomes, with two long arms fused, and is phenotypically normal.

Why is "47 chromosomes with mosaic trisomy 21" not the answer?

A mosaic trisomy 21 mother would usually show features of Down syndrome and would not explain the translocation chromosome.

Why is "46 chromosomes with a balanced reciprocal translocation between 14 an…" not the answer?

A reciprocal translocation preserves 46 chromosomes and cannot produce a der(14;21) with fused long arms.

Why is "46 chromosomes with a normal karyotype and general-population recurre…" not the answer?

Normal parental karyotypes would imply an age-related low recurrence, which does not fit the translocation seen in the child.

What concept does this question test?

It tests Genetics: Robertsonian translocation in Anatomy. Her gametes can contain an extra copy of chromosome 21 material, giving translocation Down syndrome that recurs regardless of maternal age. Genetic counselling and parental karyotyping are indicated.

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: Robertsonian translocation?

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.