Which is the following is not inherited as X-linked recessive: March 2005, September 2012, March 2013
**Core Concept**
X-linked recessive disorders are caused by mutations in genes located on the X chromosome, which are more common in males due to having only one X chromosome. Females can be carriers of these conditions, but they usually do not express the disease as they have another X chromosome with a normal gene copy.
**Why the Correct Answer is Right**
Cystic fibrosis is an autosomal recessive disorder caused by mutations in the CFTR gene on chromosome 7, not an X-linked gene. This means that both males and females are equally affected, and it is inherited in an autosomal recessive pattern. The CFTR gene codes for a chloride channel in epithelial cells, and mutations lead to abnormal ion transport, resulting in thickened mucus and respiratory complications.
**Why Each Wrong Option is Incorrect**
**Option A:** G-6-PD deficiency is an X-linked recessive disorder, caused by mutations in the G6PD gene on the X chromosome. It affects red blood cells and can lead to hemolytic anemia upon exposure to certain medications or infections.
**Option B:** Duchenne muscular dystrophy is an X-linked recessive disorder, caused by mutations in the dystrophin gene on the X chromosome. It affects muscle strength and function, leading to progressive muscle degeneration.
**Option D:** Hemophilia is also an X-linked recessive disorder, caused by mutations in the F8 or F9 genes on the X chromosome. It affects blood clotting and can lead to prolonged bleeding.
**Clinical Pearl / High-Yield Fact**
X-linked recessive disorders are more common in males due to their single X chromosome, making them more susceptible to expressing the disease. Females can be carriers but usually do not express the disease, although they can pass the mutated gene to their offspring.
**Correct Answer:**
✓ Correct Answer: C. Cystic fibrosis