NORCET 5 mains: Identify the following disorder in pedigree flowchart?

NORCET 5 mains

Pathology & Genetics

Hard

Appeared in: NORCET 5 mains

Explanation

The pedigree displays the classic hallmark of X-linked dominant inheritance.
The key feature is that an affected father transmits the trait to all of his daughters.
Simultaneously, an affected father does not transmit the trait to any of his sons.
This occurs because a father passes his single X chromosome (which carries the dominant allele) to all daughters and his Y chromosome to all sons.
The disorder appears in every generation, which is characteristic of dominant traits.

Option A: In autosomal dominant inheritance, an affected father can pass the trait to both sons and daughters. The pedigree clearly shows no father-to-son transmission.
Option B: Autosomal recessive disorders typically skip generations and often appear in offspring of unaffected carrier parents. This pedigree shows an affected parent and affected offspring in consecutive generations.
Option C: In X-linked recessive inheritance, an affected father would pass the gene to all his daughters, but they would be carriers and phenotypically unaffected (assuming the mother is not a carrier). The daughters in this pedigree are clearly affected.

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.
Pedigree analysis is a fundamental tool in genetic counseling to determine the risk of family members inheriting a genetic disorder.
Understanding inheritance patterns helps in diagnosing conditions and providing prognostic information. Examples of X-linked dominant disorders include Fragile X syndrome, Rett syndrome, and Alport syndrome.
What if? If one of the daughters in Generation II was unaffected, it would rule out X-linked dominant inheritance, as an affected father must pass his mutated X chromosome to all daughters.

How to Approach the Question

First, carefully examine the pedigree chart, identifying affected (shaded) and unaffected (unshaded) individuals, as well as their gender (square for male, circle for female) and relationships.
Look for key patterns. Does the trait appear in every generation (suggesting dominant) or does it skip generations (suggesting recessive)?
Analyze transmission based on sex. Check for father-to-son transmission. If it is present, you can rule out X-linked inheritance.
In this case, note the absence of father-to-son transmission and that the affected father passes the trait to all his daughters.
Evaluate this specific pattern against the rules for each mode of inheritance (Autosomal Dominant/Recessive, X-linked Dominant/Recessive).
The pattern of an affected father having all affected daughters and no affected sons is the definitive sign of X-linked dominant inheritance.

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 2 p. 1695-1697

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