Forensic identification from blood at a crime scene: which technique classically establishes the culprit's genetic identity?

Introduction / Context:
Forensic genetics identifies individuals by analyzing DNA variation. Historically, restriction fragment length polymorphism (RFLP) analysis produced highly discriminating DNA fingerprints from blood stains.

Given Data / Assumptions:

• A blood sample from the crime scene contains genomic DNA.
• Distinct individuals have polymorphic restriction patterns or short tandem repeat profiles.
• Classical methods relied on Southern blotting of restriction-digested DNA (RFLP).

Concept / Approach:
RFLP mapping compares restriction fragment sizes at variable loci among individuals. Matching patterns between crime scene DNA and a suspect strongly implicate identity. Although modern forensics often uses PCR-based STR profiling, RFLP represents the historically foundational, highly specific method among the options given.

Step-by-Step Solution:
Isolate DNA from the bloodstain.Digest with restriction enzymes; separate fragments by gel electrophoresis.Transfer to membrane (Southern blot) and probe polymorphic loci.Compare the banding pattern to that of suspects; near-unique matches identify the donor.

Verification / Alternative check:
Multiple independent polymorphic loci increase statistical power; probability of random match becomes vanishingly small with enough loci.

Why Other Options Are Wrong:

• Whole-genome sequencing: Overkill, not historically standard, and not among typical routine forensic workflows listed.
• PCR alone: Requires locus-specific profiling (e.g., STRs) to identify individuals; “PCR alone” is incomplete.
• Western/Northern blots: Protein or RNA analyses lack the stable, individual-specific DNA polymorphisms required.

Common Pitfalls:
Confusing method categories; RFLP (DNA) and STR-PCR are identity-based, whereas protein/RNA assays are not definitive for identity.

Final Answer:
RFLP mapping (DNA fingerprinting).