## Core Concept
The question describes a case of tooth discoloration with specific clinical findings, suggestive of a genetic disorder affecting enamel formation. The condition described is likely related to **Amelogenesis Imperfecta (AI)**, a disorder characterized by defective enamel formation.

## Why the Correct Answer is Right
Amelogenesis Imperfecta is caused by mutations in genes that encode proteins crucial for enamel formation, such as **amelogenin (AMEL1)**. The amelogenin gene is located on the X chromosome, which explains the **X-linked inheritance pattern** observed in this case, as the mother and son are both affected. The clinical features described, including mottled, opaque white to brown-yellowish discoloration of teeth, "snowcapped" appearance, and a thin layer of enamel, are consistent with AI. The positive probe test indicates the enamel is soft and prone to wear.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not correspond to the gene associated with the described condition.
* **Option B:** This option is incorrect as it does not relate to the specific genetic defect associated with Amelogenesis Imperfecta.
* **Option D:** This option is incorrect because it does not accurately represent the gene defect responsible for the clinical presentation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Amelogenesis Imperfecta** often presents with **X-linked inheritance** when associated with mutations in the **amelogenin gene (AMEL1)**, which is located on the X chromosome. This condition affects enamel formation, leading to discolored, malformed teeth. Clinicians should consider genetic counseling for families affected by this condition.

## Correct Answer Line
**Correct Answer: C. AMEL1.**