**Core Concept**
The major histocompatibility complex (MHC) is a group of genes that play a critical role in the immune system by encoding proteins responsible for presenting peptide fragments from pathogens to T-cells. The MHC gene complex is a cluster of genes that are highly polymorphic, meaning they have many different alleles (forms) in the population.

**Why the Correct Answer is Right**
The MHC gene complex is located on chromosome 6 in humans. Specifically, the HLA (human leukocyte antigen) region, which is part of the MHC complex, is situated on the short arm of chromosome 6 (6p21.3). The HLA region encodes the class I and class II MHC molecules, which are essential for the presentation of antigens to T-cells. The presence of multiple alleles at the HLA locus allows for a high degree of diversity in the immune response, enabling the body to recognize and respond to a wide range of pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the MHC gene complex is not located on chromosome 1.
**Option B:** Incorrect because the MHC gene complex is not located on chromosome 9.
**Option C:** Incorrect because the MHC gene complex is not located on chromosome 12.

**Clinical Pearl / High-Yield Fact**
Understanding the MHC gene complex is crucial for immunogenetics and transplantation medicine. The HLA typing of donors and recipients is essential for matching organ transplants and preventing graft rejection. A mismatch between the HLA types of the donor and recipient can lead to a severe immune response and graft failure.

**Correct Answer:** D. Chromosome 6.