A 12 year old boy presented with progressive muscular weakness showing the following finding, and it was found that the child is unable to get up from squatting position. He was diagnosed with hereditary muscular disorder. What is the mode of inheritance in this disorder?
Looking at the options, the correct answer is X-linked recessive. The other options would be autosomal recessive, autosomal dominant, or maybe another type. Let me think about why the others are wrong. Autosomal recessive would require two copies, so females could be carriers but less likely to show symptoms. Autosomal dominant would affect both sexes equally and require one copy. Mitochondrial inheritance is rare here. Since the patient is male and the disorder is X-linked, the mode is X-linked recessive.
The clinical pearl here is to remember Gower's sign as a red flag for DMD. Also, the age of onset is typically between 3-5 years, but symptoms can progress to the point of presentation at 12. The inheritance pattern is crucial for family counseling and genetic testing.
**Core Concept**
The scenario describes Duchenne muscular dystrophy (DMD), an X-linked recessive disorder caused by dystrophin deficiency. The inability to rise from a squat (Gower’s sign) is a hallmark clinical feature. X-linked recessive inheritance explains why males are predominantly affected, with females typically being asymptomatic carriers.
**Why the Correct Answer is Right**
DMD is caused by mutations in the *DMD* gene on the X chromosome (Xp21.2). Males inherit the defective X chromosome from their carrier mothers and lack a compensatory normal allele. Dystrophin deficiency leads to progressive muscle degeneration. The clinical presentation aligns with DMD’s typical onset between ages 3–5, with Gower’s sign appearing as weakness worsens. The X-linked pattern is confirmed by the male predominance and family history patterns.
**Why Each Wrong Option is Incorrect**
**Option A:** *Autosomal dominant* inheritance would affect both sexes equally and require only one affected parent to transmit the disorder.
**Option B:** *Autosomal recessive* disorders require two defective alleles; females would be more likely to inherit two copies in consanguineous families.
**Option C:** *Mitochondrial inheritance* affects both genders but follows maternal transmission, unrelated to dystrophinopathy.
**Clinical Pearl / High-Yield Fact**
Gower’s sign (pushing off the legs to stand from a squat) is a diagnostic red flag for DMD. Remember: X-linked recessive disorders like DMD, hemophilia, and color blindness follow a crisscross inheritance pattern—carrier mothers pass mutations to affected sons.
**Correct Answer: C. X-linked recessive**