A 4-year-old boy presents with progressive proximal lower-limb weakness, toe-walking, and frequent falls over the past year. His mother reports he walked normally until age 2 years. Serum creatine kinase is markedly elevated at 45,000 IU/L. Genetic testing confirms a deletion in the DMD gene on chromosome Xp21.1 with loss of dystrophin expression. The inheritance pattern marked **A** in the pedigree diagram shows the characteristic X-linked recessive transmission. Which of the following best explains why this boy's father is clinically unaffected despite carrying the same mutation?
A. The father inherited the mutant allele on his Y chromosome, which does not carry the DMD gene
B. The father inherited the mutant allele on his single X chromosome but males do not express X-linked recessive traits
C. The father inherited the mutant allele from his mother, who was a symptomatic carrier
D. The father does not carry the mutation; the boy inherited it exclusively from his mother through X-linked recessive transmission
Explanation
Why option 4 is right
In X-linked recessive inheritance (pattern A), affected males inherit the mutant allele exclusively from their carrier mothers. An affected father CANNOT transmit his mutant X chromosome to his sons—he passes his only X to daughters (who become obligate carriers) and his Y to sons. Therefore, this boy's father is clinically unaffected because he does not carry the DMD mutation; the boy inherited the deletion from his carrier mother. This is the hallmark of X-linked recessive inheritance: no male-to-male transmission. [AAN Care Considerations for DMD; Birnkrant et al Lancet Neurol 2018]
Why each distractor is wrong
Option 1: Symptomatic carriers are extremely rare in DMD (only in Turner syndrome 45,X or skewed X-inactivation). Standard carrier females are asymptomatic or mildly affected. This misrepresents the typical inheritance.
Option 2: The Y chromosome does not carry the DMD gene; the DMD gene is located on chromosome Xp21.1. Males have one X and one Y. This is factually incorrect.
Option 3: This reverses the genetic principle. Males with a single mutant X allele ARE affected in X-linked recessive disorders because they have no second X to compensate. Males express X-linked recessive traits, not avoid them.
High-YieldNEET PG
X-linked recessive inheritance = affected males connected through carrier mothers, NO male-to-male transmission (affected father's sons are always unaffected; his daughters are always obligate carriers).
AAN Care Considerations for DMD; Birnkrant et al Lancet Neurol 2018
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