**Core Concept**
In alpha-thalassemia, the severity of the disorder is determined by the number of gene loci affected. The gene loci responsible for alpha-globin production are located on chromosomes 16. Deletions or mutations in these gene loci lead to reduced or absent production of alpha-globin chains, resulting in various forms of alpha-thalassemia.

**Why the Correct Answer is Right**
The severity of alpha-thalassemia is classified based on the number of gene loci affected. Hb Barts is produced when there is a complete loss of alpha-globin production, which occurs when all four gene loci are affected. This is because the beta-globin chains produced in excess form tetramers, resulting in the production of Hb Barts (γ4). The correct classification is as follows:
- Two gene loci affected: β-thalassemia trait
- Three gene loci affected: Hemoglobin H disease
- Four gene loci affected: Hb Barts hydrops fetalis

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the severity of alpha-thalassemia is directly correlated with the number of gene loci affected. This understanding is crucial for diagnosing and managing patients with alpha-thalassemia.

**Correct Answer: D. Four gene loci affected. Hb Barts is produced when all four gene loci are affected, resulting in a complete loss of alpha-globin production.**