**Core Concept**
The retinoblastoma gene is a tumor suppressor gene that plays a crucial role in regulating the cell cycle and preventing uncontrolled cell growth in the retina. Mutations in this gene can lead to the development of retinoblastoma, a rare form of eye cancer that primarily affects children.

**Why the Correct Answer is Right**
The retinoblastoma gene is located on the long arm of chromosome 13 (13q14). This gene encodes a protein called pRb, which binds to and inhibits the E2F transcription factor, thereby preventing the cell cycle from progressing from the G1 phase to the S phase. When the retinoblastoma gene is mutated, the pRb protein is inactivated, allowing uncontrolled cell growth and tumor formation. The retinoblastoma protein also interacts with other proteins, such as p53, to regulate cell cycle progression and apoptosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the retinoblastoma gene is not located on chromosome 1.

**Option B:** This option is incorrect as the retinoblastoma gene is not located on chromosome 11.

**Option D:** This option is incorrect as the retinoblastoma gene is not located on chromosome 21.

**Clinical Pearl / High-Yield Fact**
The retinoblastoma gene is a classic example of a tumor suppressor gene, and its mutation can lead to the development of various types of cancer, including retinoblastoma, osteosarcoma, and breast cancer. Understanding the role of tumor suppressor genes is crucial in understanding the pathogenesis of cancer.

**Correct Answer:** C. 13q14.