Genetics: penetrance and expressivity: practice question

A Biochemistry 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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Biochemistry · Genetics: penetrance and expressivity

Sample item

In a family with an autosomal dominant disorder, genetic testing identifies 20 individuals who carry the pathogenic variant. Of these, 17 have clinical features and 3 are completely normal, but one of the unaffected carriers has an affected child. Which statement is correct?

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

The answer and the reasoning

A. Penetrance is 85 percent, and the unaffected carriers illustrate reduced (incomplete) penetrance

Penetrance is the proportion of individuals with the genotype who show any phenotype: 17 of 20 = 85 percent. Carriers with no features despite the variant show reduced penetrance and can still pass the variant on, so the disorder appears to skip a generation. Variable expressivity describes differences in severity among those who are affected.

Option by option

  • A. Penetrance is 85 percent, and the unaffected carriers illustrate reduced (incomplete) penetrance (correct)

    Seventeen of twenty carriers affected gives 85 percent penetrance, and non-manifesting carriers show incomplete penetrance.

  • B. Penetrance is 85 percent, and the unaffected carriers illustrate variable expressivity

    The 85 percent figure is right, but expressivity refers to severity among affected people, not to the absence of any phenotype.

  • C. Penetrance is 15 percent, and the unaffected carriers illustrate variable expressivity

    Fifteen percent is the wrong calculation, and non-manifesting carriers are not an example of expressivity.

  • D. Penetrance is 15 percent, and the unaffected carriers illustrate reduced penetrance

    Fifteen percent is the non-penetrant fraction and not the penetrance.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Penetrance is 85 percent, and the unaffected carriers illustrate reduced (incomplete) penetrance. Penetrance is the proportion of individuals with the genotype who show any phenotype: 17 of 20 = 85 percent.

Why is "Penetrance is 85 percent, and the unaffected carriers illustrate vari…" not the answer?

The 85 percent figure is right, but expressivity refers to severity among affected people, not to the absence of any phenotype.

Why is "Penetrance is 15 percent, and the unaffected carriers illustrate vari…" not the answer?

Fifteen percent is the wrong calculation, and non-manifesting carriers are not an example of expressivity.

Why is "Penetrance is 15 percent, and the unaffected carriers illustrate redu…" not the answer?

Fifteen percent is the non-penetrant fraction and not the penetrance.

What concept does this question test?

It tests Genetics: penetrance and expressivity in Biochemistry. Carriers with no features despite the variant show reduced penetrance and can still pass the variant on, so the disorder appears to skip a generation. Variable expressivity describes differences in severity among those who are affected.

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: penetrance and expressivity?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.