Medical genetics: uniparental disomy: 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 · Medical genetics: uniparental disomy

Sample item

A child has cystic fibrosis. Only the mother is a carrier, the father is not, and paternity is confirmed. Genotyping shows the child is homozygous for the mother's mutant CFTR allele and for every maternal marker on chromosome 7, with a normal karyotype. Which mechanism explains this?

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

The answer and the reasoning

B. Maternal uniparental isodisomy of chromosome 7

Uniparental disomy is inheritance of both copies of a chromosome from one parent. In isodisomy, two identical copies of one parental chromosome are present, so a recessive mutation in a single carrier parent can become homozygous. In heterodisomy the two different parental copies are present, and the child would be a carrier.

Option by option

  • A. Paternal uniparental isodisomy of chromosome 7

    Paternal disomy would carry the father's alleles, and the father is not a carrier.

  • B. Maternal uniparental isodisomy of chromosome 7 (correct)

    Correct. Two identical copies of the maternal chromosome unmask the recessive allele.

  • C. Maternal uniparental heterodisomy of chromosome 7

    Heterodisomy would give one mutant and one normal maternal allele, making the child a carrier, not affected.

  • D. Balanced Robertsonian translocation involving chromosome 7

    A Robertsonian translocation involves acrocentric chromosomes and does not explain homozygosity.

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 B, Maternal uniparental isodisomy of chromosome 7. Uniparental disomy is inheritance of both copies of a chromosome from one parent.

Why is "Paternal uniparental isodisomy of chromosome 7" not the answer?

Paternal disomy would carry the father's alleles, and the father is not a carrier.

Why is "Maternal uniparental heterodisomy of chromosome 7" not the answer?

Heterodisomy would give one mutant and one normal maternal allele, making the child a carrier, not affected.

Why is "Balanced Robertsonian translocation involving chromosome 7" not the answer?

A Robertsonian translocation involves acrocentric chromosomes and does not explain homozygosity.

What concept does this question test?

It tests Medical genetics: uniparental disomy in Anatomy. In isodisomy, two identical copies of one parental chromosome are present, so a recessive mutation in a single carrier parent can become homozygous. In heterodisomy the two different parental copies are present, and the child would be a carrier.

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 Medical genetics: uniparental disomy?

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.