Chromosomal disorders: Robertsonian translocation: 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 · Chromosomal disorders: Robertsonian translocation

Sample item

A couple has a child with Down syndrome who has 46 chromosomes, the extra chromosome 21 being fused to chromosome 14 (translocation Down syndrome). Karyotyping of the parents is advised. What would the carrier parent show?

A47 chromosomes with trisomy 21B46 chromosomes with a normal karyotype and gonadal mosaicism onlyC45 chromosomes with one fused 14;21 chromosome, and a normal phenotypeD46 chromosomes with a reciprocal translocation between chromosomes 14 and 21

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

The answer and the reasoning

C. 45 chromosomes with one fused 14;21 chromosome, and a normal phenotype

A Robertsonian translocation fuses the long arms of two acrocentric chromosomes (13, 14, 15, 21 and 22) with loss of the short arms, which carry only redundant ribosomal genes. A carrier has 45 chromosomes and is phenotypically normal, but gametes can carry the translocated 21 in addition to a normal 21, producing a child with Down syndrome. Unlike trisomy from nondisjunction, this form carries a significant recurrence risk and does not depend on maternal age.

Option by option

A. 47 chromosomes with trisomy 21
Trisomy 21 would give a count of 47 and would cause Down syndrome in the parent.
B. 46 chromosomes with a normal karyotype and gonadal mosaicism only
A normal karyotype would not explain familial recurrence risk in a translocation case.
C. 45 chromosomes with one fused 14;21 chromosome, and a normal phenotype (correct)
The carrier has a balanced fusion chromosome and a chromosome count of 45.
D. 46 chromosomes with a reciprocal translocation between chromosomes 14 and 21
A reciprocal translocation preserves 46 chromosomes and is a different rearrangement.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, 45 chromosomes with one fused 14;21 chromosome, and a normal phenotype. A Robertsonian translocation fuses the long arms of two acrocentric chromosomes (13, 14, 15, 21 and 22) with loss of the short arms, which carry only redundant ribosomal genes.

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

Trisomy 21 would give a count of 47 and would cause Down syndrome in the parent.

Why is "46 chromosomes with a normal karyotype and gonadal mosaicism only" not the answer?

A normal karyotype would not explain familial recurrence risk in a translocation case.

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

A reciprocal translocation preserves 46 chromosomes and is a different rearrangement.

What concept does this question test?

It tests Chromosomal disorders: Robertsonian translocation in Biochemistry. A carrier has 45 chromosomes and is phenotypically normal, but gametes can carry the translocated 21 in addition to a normal 21, producing a child with Down syndrome. Unlike trisomy from nondisjunction, this form carries a significant recurrence risk and does not depend on maternal age.

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 Chromosomal disorders: Robertsonian translocation?

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.