In the following human pedigree, the filled symbols represent the affected individuals. Identify the type of… - NORCET 3 - 2022 (Shift-1)

NORCET 3 - 2022 (Shift-1)

Pathology & Genetics

Hard

Appeared in: NORCET 3 - 2022 (Shift-1)

Explanation

The key to identifying the inheritance pattern is the offspring of the affected father (II-5) and unaffected mother (II-4).
This couple has two affected daughters and two unaffected sons. An affected father transmitting the trait to all his daughters and none of his sons is the classic hallmark of X-linked dominant inheritance.
This occurs because a father passes his only X chromosome (which carries the dominant allele) to all his daughters, making them affected. He passes his Y chromosome to his sons, so they cannot inherit the trait from him.
The trait also appears in every generation, which is consistent with a dominant pattern of inheritance.

Option B: This is ruled out because the affected mother in generation I (I-1) has an unaffected son (II-3). In X-linked recessive inheritance, an affected mother would pass the trait to all of her sons.
Option C: While this pattern also shows vertical transmission (appearing in every generation), it does not typically result in an affected father passing the trait to 100% of his daughters and 0% of his sons. In autosomal dominant inheritance, an affected parent has a 50% chance of passing the trait to each child, regardless of sex.
Option D: This pattern is characterized by skipping generations, which is not seen here as the trait appears in all three generations. Also, in recessive inheritance, affected individuals typically have unaffected (carrier) parents.

Visual 1: Diagram: A flowchart for systematic pedigree analysis, starting with checking for dominant/recessive patterns and then for autosomal/X-linked patterns.
Visual 2: Infographic: A visual comparison of the four main Mendelian inheritance patterns (Autosomal Dominant, Autosomal Recessive, X-linked Dominant, X-linked Recessive) with example pedigrees for each.

Clinical Relevance

Nursing practice connection: Knowing Pedigree analysis and modes of genetic inheritance helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Understanding pedigree analysis is crucial for genetic counseling, allowing nurses and physicians to assess disease risk, predict recurrence in a family, and guide decisions about genetic testing.
Examples of X-linked dominant disorders include Vitamin D-resistant rickets, Rett syndrome, and Alport syndrome.
What if... the affected father (II-5) had an affected son? This single observation would definitively rule out X-linked inheritance and make Autosomal Dominant the correct answer, as it would demonstrate male-to-male transmission.

How to Approach the Question

First, observe the overall pattern. Does the trait appear in every generation? Yes (I, II, III). This suggests a dominant inheritance pattern.
Next, look for clues to distinguish between autosomal and X-linked inheritance.
Focus on the most informative part of the pedigree: an affected parent with multiple children.
Examine the offspring of the affected father (II-5) and unaffected mother (II-4). Note that all daughters are affected, and all sons are unaffected.
Recall the rules of X-linked inheritance: fathers pass their X chromosome to all daughters and their Y chromosome to sons. The observed pattern perfectly matches X-linked dominant inheritance.
Confirm this by checking for contradictions elsewhere. For example, an affected mother (I-1) can have an unaffected son (II-3), which is consistent with her being heterozygous.

Concept Tested & Keywords

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