Identification: Y-chromosome STR typing: 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: Y-chromosome STR typing

Sample item

A vaginal swab from a sexual assault contains very little male DNA mixed with a large amount of the victim's epithelial DNA, and no sperm are seen because the suspect is thought to be vasectomised. Which analysis can best detect the male contributor?

AY-chromosome STR typingBMitochondrial DNA sequencingCABO secretor testing of the swabDStandard autosomal STR typing alone

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

The answer and the reasoning

A. Y-chromosome STR typing

Y-STR primers amplify only male-specific loci, so the overwhelming female DNA is not amplified. The profile is shared by paternal male relatives, so it narrows rather than uniquely identifies. Autosomal typing is swamped by the female component, and mitochondrial DNA is maternally inherited.

Option by option

A. Y-chromosome STR typing (correct)
Correct. Y-STR typing targets male DNA even in a large female background.
B. Mitochondrial DNA sequencing
Mitochondrial DNA is inherited from the mother and is not specific for the male contributor.
C. ABO secretor testing of the swab
ABO secretor typing is not individualising and is overshadowed by the victim's own secretions.
D. Standard autosomal STR typing alone
Autosomal STR typing would be dominated by the victim's DNA in this mixture.

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 A, Y-chromosome STR typing. Y-STR primers amplify only male-specific loci, so the overwhelming female DNA is not amplified.

Why is "Mitochondrial DNA sequencing" not the answer?

Mitochondrial DNA is inherited from the mother and is not specific for the male contributor.

Why is "ABO secretor testing of the swab" not the answer?

ABO secretor typing is not individualising and is overshadowed by the victim's own secretions.

Why is "Standard autosomal STR typing alone" not the answer?

Autosomal STR typing would be dominated by the victim's DNA in this mixture.

What concept does this question test?

It tests Identification: Y-chromosome STR typing in Forensic Medicine. The profile is shared by paternal male relatives, so it narrows rather than uniquely identifies. Autosomal typing is swamped by the female component, and mitochondrial DNA is maternally inherited.

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: Y-chromosome STR typing?

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.