Cancer genetics — Which blotting technique is most directly used to detect DNA-level changes that can identify a defective gene implicated in cancer predisposition or development?

Introduction:
Diagnosing or researching genetic defects associated with cancer often requires assays that reveal DNA-level alterations such as insertions, deletions, or rearrangements. Different blotting techniques interrogate different biomolecules. This question checks whether you can match the technique to the biological target relevant for gene defects.

Given Data / Assumptions:

• Southern blot analyzes DNA fragments separated by size and hybridized with labeled probes.
• Northern blot measures RNA abundance (expression), not genomic changes directly.
• Western blot detects proteins; band shifts reflect size/abundance, not primary DNA mutations.

Concept / Approach:
Because many cancer-associated defects are structural or sequence changes in DNA, a hybridization assay at the DNA level is appropriate. Southern blotting reveals altered restriction patterns, gene rearrangements, and copy-number changes when probed with gene-specific sequences.

Step-by-Step Solution:

Verification / Alternative check:
Classic identification of oncogene amplification or immunoglobulin/T-cell receptor gene rearrangements used Southern blot; modern methods include PCR and next-generation sequencing but the principle remains DNA-level detection.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing expression changes (RNA/protein) with genomic mutations; assuming all blots are interchangeable.

Final Answer:
Southern blot.