& x20;Which statement correctly describes incomplete penetrance? - INI-CET EXAM -2026

INI-CET EXAM -2026

Pathology & Genetics (E5)

Easy

Appeared in: INI-CET EXAM -2026

Explanation

Incomplete penetrance occurs when individuals with a disease-causing genotype do not develop the disease phenotype.
This leads to the appearance of the disease 'skipping' a generation in a pedigree.
For a dominant disorder, this results in a pattern where affected individuals appear in non-consecutive or alternate generations (e.g., grandparent and grandchild are affected, but the parent is not).
Therefore, the statement that alternate generations are affected is the best description of how incomplete penetrance manifests in a family history.

Option A: This is incorrect because inheritance is probabilistic. For a dominant trait, an affected parent has a 50% chance of passing the gene to a child. Incomplete penetrance means that even if the child inherits the gene, they may not be affected.
Option B: This is the opposite of what can happen. An unaffected carrier due to incomplete penetrance can still pass the disease-causing allele to their children, who may then manifest the disease.
Option C: The term 'entirely spared' is too absolute and therefore incorrect. Incomplete penetrance applies to individuals, not necessarily to an entire generation. It's possible for some siblings in a generation to be affected while others are not, and for an unaffected individual to have an affected child.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain The concept being tested is the definition and manifestation of incomplete penetrance in genetic inheritance patterns as background academic context rather than a clinical decision trigger.
Understanding incomplete penetrance is vital for genetic counseling. A nurse must be able to explain to a patient that having a disease-causing gene (like BRCA1/2 for breast cancer) does not guarantee they will develop the disease, but it does increase their risk significantly.
It complicates risk assessment. When taking a family history, a nurse might see a 'skipped' generation and must consider incomplete penetrance as a possible explanation, rather than ruling out a genetic link.
What if? If a genetic test shows a patient has a mutation for a condition with incomplete penetrance, but they have no symptoms, the nursing plan focuses on surveillance and risk-reduction strategies (e.g., more frequent screenings) rather than treatment for an active disease.

How to Approach the Question

First, identify the core genetic concept in the question, which is 'incomplete penetrance'.
Recall the definition: An individual has the gene (genotype) but does not show the trait (phenotype).
Visualize how this would affect a family tree (pedigree). The trait would seem to disappear in one person and reappear in their offspring.
This phenomenon is commonly called 'skipping a generation'.
Evaluate the options based on this understanding. Option A and B are fundamentally incorrect about genetic transmission.
Compare C and D. 'Alternate generations' (D) is a standard way to describe the 'skipping' pattern, whereas 'entirely spared' (C) is an inaccurate overstatement.

Concept Tested & Keywords

Concept Tested: The concept being tested is the definition and manifestation of incomplete penetrance in genetic inheritance patterns.
Stem keywords: incomplete penetrance, genetics, pedigree
Lead-in keywords: correctly describes

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E6 Medicine Harrison 22e Part 2 p. 1693-1695

E6 Nelson Textbook of Pediatrics(2024) — Volume 1 p. 779-781