## **Core Concept**
X-linked diseases are genetic disorders caused by mutations in genes on the X chromosome. These conditions predominantly affect males, who have one X and one Y chromosome, as they have no second X chromosome to compensate for the defective gene. Females, on the other hand, can be carriers of X-linked diseases, having one normal and one mutated gene on their two X chromosomes.

## **Why the Correct Answer is Right**
The correct answer, **β-thalassemia**, is not an X-linked disease; it is an autosomal recessive disorder. β-thalassemia is caused by mutations in the HBB gene on chromosome 11, which codes for the beta-globin subunit of hemoglobin. This condition leads to reduced or absent production of the beta-globin chains of hemoglobin, resulting in various degrees of anemia.

## **Why Each Wrong Option is Incorrect**
- **Option A: Androgen insensitivity syndrome** is an X-linked condition. It results from mutations in the androgen receptor gene on the X chromosome, leading to resistance to male hormones (androgens) and causing individuals with XY chromosomes to develop as females.
- **Option B: Duchenne muscular dystrophy** is an X-linked recessive disorder. It is caused by mutations in the dystrophin gene on the X chromosome, leading to progressive muscle degeneration and weakness.
- **Option C: Hemophilia** is also an X-linked recessive disorder. It is primarily caused by mutations in the F8 gene (Hemophilia A) or F9 gene (Hemophilia B) on the X chromosome, affecting the production of clotting factors VIII and IX, respectively.
- **Option E: Wilson's disease** is an autosomal recessive disorder, not X-linked. It is caused by mutations in the ATP7B gene on chromosome 13, which plays a critical role in regulating the transport of copper in the liver.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that X-linked diseases often present differently in males versus females due to X-chromosome inactivation patterns in females. Males are more frequently and severely affected because they have only one X chromosome. Females can be asymptomatic carriers or exhibit milder symptoms due to X-chromosome inactivation skewing.

## **Correct Answer: D. β-thalassemia**