Genetics: carrier risk calculation: 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: carrier risk calculation

Sample item

A healthy woman has a brother with an autosomal recessive disorder, and her parents are healthy. Her unrelated partner has no family history, and the carrier frequency in his population is 1 in 50. What is the probability that their first child is affected?

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

The answer and the reasoning

A. 1 in 300

Her parents are obligate carriers, so as an unaffected sibling she has a 2 in 3 chance of being a carrier. The partner has a 1 in 50 chance. If both are carriers the child has a 1 in 4 chance of being affected. The risk is 2/3 x 1/50 x 1/4 = 2/600 = 1 in 300.

Option by option

  • A. 1 in 300 (correct)

    Correct. 2/3 x 1/50 x 1/4 = 1 in 300.

  • B. 1 in 150

    1 in 150 results from using 1/2 instead of 1/4 for the chance of an affected child.

  • C. 1 in 200

    1 in 200 results from assuming she is a certain carrier (1 x 1/50 x 1/4).

  • D. 1 in 600

    1 in 600 results from using 1/3 instead of 2/3 for her carrier chance.

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 A, 1 in 300. Her parents are obligate carriers, so as an unaffected sibling she has a 2 in 3 chance of being a carrier.

Why is "1 in 150" not the answer?

1 in 150 results from using 1/2 instead of 1/4 for the chance of an affected child.

Why is "1 in 200" not the answer?

1 in 200 results from assuming she is a certain carrier (1 x 1/50 x 1/4).

Why is "1 in 600" not the answer?

1 in 600 results from using 1/3 instead of 2/3 for her carrier chance.

What concept does this question test?

It tests Genetics: carrier risk calculation in Anatomy. The partner has a 1 in 50 chance. If both are carriers the child has a 1 in 4 chance of being affected. The risk is 2/3 x 1/50 x 1/4 = 2/600 = 1 in 300.

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: carrier risk calculation?

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.