**Core Concept**
FISH (Fluorescence In Situ Hybridization) is a molecular cytogenetic technique used to detect and locate specific DNA sequences on chromosomes. It involves the hybridization of a fluorescently labeled probe to the target gene locus, allowing for its visualization under a fluorescence microscope.

**Why the Correct Answer is Right**
FISH is a versatile technique used to identify specific genes, detect chromosomal abnormalities, and study gene expression. The process involves denaturing the chromosomal DNA, followed by the hybridization of a labeled probe that is complementary to the target gene sequence. The fluorescent label allows for the visualization of the probe binding to the target gene locus, enabling its precise localization. This technique is particularly useful in identifying genetic disorders, cancer diagnosis, and studying gene expression patterns.

**Why Each Wrong Option is Incorrect**
**Option B:** CGH (Comparative Genomic Hybridization) is a technique used to detect copy number variations in the genome, but it does not specifically locate known gene loci.
**Option C:** Chromosome painting is a technique used to visualize entire chromosomes, but it is not used to locate specific gene loci.
**Option D:** RT-PCR (Reverse Transcription Polymerase Chain Reaction) is a technique used to amplify specific mRNA sequences, but it does not involve the localization of gene loci.

**Clinical Pearl / High-Yield Fact**
FISH is a valuable tool in molecular diagnostics, allowing for the rapid and accurate identification of specific gene mutations associated with various genetic disorders. This technique has revolutionized the field of genetics and has significant implications for personalized medicine.

**✓ Correct Answer: A. FISH**