**Core Concept**
The Rh blood group system is a complex immunohematological system that involves multiple antigens and their corresponding antibodies. The RhD antigen is a crucial component of this system, and its presence or absence determines an individual's Rh blood type.

**Why the Correct Answer is Right**
The RhD antigen is encoded by the RHD gene, which is located on the short arm of chromosome 1 (1p36.11-p36.13). This gene is a member of the immunoglobulin superfamily and encodes a transmembrane protein that is expressed on the surface of red blood cells. The RHD gene is responsible for the production of the RhD antigen, which is recognized by the immune system as foreign and triggers an immune response in Rh-negative individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the RhD antigen is not encoded by a gene on chromosome 2.
**Option B:** This option is incorrect because the RhD antigen is not encoded by a gene on chromosome 3.
**Option C:** This option is incorrect because the RhD antigen is not encoded by a gene on chromosome 4.

**Clinical Pearl / High-Yield Fact**
The Rh blood type is an important consideration in transfusion medicine, as Rh-positive blood can trigger an immune response in Rh-negative individuals, leading to hemolysis and other complications. Understanding the genetics of the Rh blood group system is crucial for ensuring safe blood transfusions.

**Correct Answer:** D. 1