Identification: interpretation of DNA mixtures: practice question

A Forensic Medicine 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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Forensic Medicine · Identification: interpretation of DNA mixtures

Sample item

An STR profile from a vaginal swab of an assault survivor shows, at most loci, three or four alleles of comparable height in addition to the survivor's own alleles. What is the best interpretation?

AMitochondrial heteroplasmyBA mixture of DNA from more than one contributorCStutter peaks from polymerase slippageDDegradation of the template DNA

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

The answer and the reasoning

B. A mixture of DNA from more than one contributor

More than two alleles of comparable height at many loci indicates a mixture of contributors, here the survivor and one or more other persons. Stutter is a small peak one repeat unit shorter than the true allele. Degradation reduces large loci rather than adding alleles.

Option by option

A. Mitochondrial heteroplasmy
Heteroplasmy concerns mitochondrial DNA and not nuclear STR loci.
B. A mixture of DNA from more than one contributor (correct)
Correct. Multiple alleles of comparable height at most loci indicate several contributors.
C. Stutter peaks from polymerase slippage
Stutter peaks are small, one repeat shorter and occur at a minor fraction of the main allele.
D. Degradation of the template DNA
Degradation causes loss of larger loci and does not add alleles.

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 Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is B, A mixture of DNA from more than one contributor. More than two alleles of comparable height at many loci indicates a mixture of contributors, here the survivor and one or more other persons.

Why is "Mitochondrial heteroplasmy" not the answer?

Heteroplasmy concerns mitochondrial DNA and not nuclear STR loci.

Why is "Stutter peaks from polymerase slippage" not the answer?

Stutter peaks are small, one repeat shorter and occur at a minor fraction of the main allele.

Why is "Degradation of the template DNA" not the answer?

Degradation causes loss of larger loci and does not add alleles.

What concept does this question test?

It tests Identification: interpretation of DNA mixtures in Forensic Medicine. Stutter is a small peak one repeat unit shorter than the true allele. Degradation reduces large loci rather than adding alleles.

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 Identification: interpretation of DNA mixtures?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine 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.