Genetics: linkage analysis: 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: linkage analysis

Sample item

A family study of an autosomal dominant disorder gives a maximum LOD score of 3.0 between the disease locus and a DNA marker. How is this result interpreted?

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

The answer and the reasoning

C. Odds of 1000 to 1 in favour of linkage

The LOD score is the base 10 logarithm of the odds that two loci are linked versus unlinked. A score of 3 means 10 to the power 3, or odds of 1000 to 1, and is the conventional threshold to accept linkage (a score of minus 2 excludes it). The LOD score does not itself give the recombination fraction, which is estimated separately as the value at which the score is maximal.

Option by option

  • A. Three times greater probability of linkage than of non-linkage

    Three times would be a LOD of about 0.5; a LOD of 3 is a thousand-fold.

  • B. A 3% probability that the result occurred by chance

    The score is a log of odds, not a probability of chance, and 1000 to 1 is much stronger than 3%.

  • C. Odds of 1000 to 1 in favour of linkage (correct)

    Correct. A LOD of 3 equals odds of 1000 to 1 in favour of linkage.

  • D. A recombination fraction of 3%

    The recombination fraction is a separate estimate, not the LOD score value.

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.

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Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is C, Odds of 1000 to 1 in favour of linkage. The LOD score is the base 10 logarithm of the odds that two loci are linked versus unlinked.

Why is "Three times greater probability of linkage than of non-linkage" not the answer?

Three times would be a LOD of about 0.5; a LOD of 3 is a thousand-fold.

Why is "A 3% probability that the result occurred by chance" not the answer?

The score is a log of odds, not a probability of chance, and 1000 to 1 is much stronger than 3%.

Why is "A recombination fraction of 3%" not the answer?

The recombination fraction is a separate estimate, not the LOD score value.

What concept does this question test?

It tests Genetics: linkage analysis in Anatomy. A score of 3 means 10 to the power 3, or odds of 1000 to 1, and is the conventional threshold to accept linkage (a score of minus 2 excludes it). The LOD score does not itself give the recombination fraction, which is estimated separately as the value at which the score is maximal.

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 Genetics: linkage analysis?

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.