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 miscarriages and a child with Down syndrome whose karyotype is 46,XY,rob(14;21),+21. If the mother carries the balanced translocation, how many chromosomes does she have?

A45B44C46D47

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

The answer and the reasoning

A. 45

A Robertsonian translocation fuses the long arms of two acrocentric chromosomes at the centromere and the short arms are lost, a loss without phenotypic effect because they carry redundant ribosomal genes. The balanced carrier therefore has 45 chromosomes with one fused 14;21 chromosome. Her gametes may carry the translocated chromosome plus a normal 21, producing translocation Down syndrome.

Option by option

A. 45 (correct)
Correct. Fusion of two chromosomes into one reduces the count from 46 to 45.
B. 44
A count of 44 would require loss of two whole chromosomes.
C. 46
A count of 46 would apply to a person with a normal karyotype, or to the affected child.
D. 47
A count of 47 would indicate a trisomy such as trisomy 21 with free chromosomes.

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.

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is A, 45. A Robertsonian translocation fuses the long arms of two acrocentric chromosomes at the centromere and the short arms are lost, a loss without phenotypic effect because they carry redundant ribosomal genes.

Why is "44" not the answer?

A count of 44 would require loss of two whole chromosomes.

Why is "46" not the answer?

A count of 46 would apply to a person with a normal karyotype, or to the affected child.

Why is "47" not the answer?

A count of 47 would indicate a trisomy such as trisomy 21 with free chromosomes.

What concept does this question test?

It tests Genetics: Robertsonian translocation in Anatomy. The balanced carrier therefore has 45 chromosomes with one fused 14;21 chromosome. Her gametes may carry the translocated chromosome plus a normal 21, producing translocation Down syndrome.

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: 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.