Fingerprints are unique patterns of ridges and valleys on human fingertips, formed in the womb, with no two individuals, not even identical twins, having the exact same set. Beyond their visible patterns, fingerprints also leave a biological signature. Deoxyribonucleic acid, or DNA, is the fundamental blueprint for life, carrying genetic instructions for all living organisms. The question then arises whether this genetic information can be retrieved from the impressions left by our fingers.
How DNA is Left on Surfaces
While a visible fingerprint is an impression of friction ridges, the DNA comes from trace evidence transferred from the skin. This trace evidence primarily consists of shed skin cells (epithelial cells), sweat, and oils.
Humans constantly shed skin cells containing DNA. When a hand touches an object, these cells and secretions from sweat and sebaceous glands transfer to the surface. Even a light touch can leave enough cellular material for DNA analysis. This biological residue, rather than the physical print, is the source material.
From Print to Profile: DNA Extraction
Obtaining a usable DNA sample from trace evidence involves laboratory procedures. Forensic scientists collect potential DNA at a crime scene, often by swabbing or scraping, then transport it for processing.
The first step is DNA extraction, chemically breaking open cells to release and purify DNA. Since quantities are often small, Polymerase Chain Reaction (PCR) amplifies specific DNA regions, making millions of copies. The amplified DNA then undergoes profiling, commonly using Short Tandem Repeat (STR) analysis. STR analysis examines highly variable, repetitive DNA sequences to create a unique genetic profile.
Forensic scientists primarily analyze nuclear DNA (nDNA), found in cell nuclei and inherited from both parents, providing a unique profile (except for identical twins). If nDNA is degraded or limited, mitochondrial DNA (mtDNA) can be analyzed. MtDNA, inherited solely from the mother and present in many copies per cell, is more robust but less unique for identification.
Real-World Uses and Challenges
Obtaining DNA from fingerprints has numerous practical applications, particularly in forensic science. It links individuals to crime scenes by comparing DNA profiles from trace evidence to suspects or existing databases. This technique also aids paternity testing, human remains identification, and historical investigations.
Despite its utility, obtaining DNA from fingerprints presents challenges. The quantity of DNA is often extremely low, sometimes requiring “low copy number” (LCN) techniques that amplify DNA from as few as a few cells. Such minute quantities are prone to degradation from environmental factors like heat and moisture, leading to partial or unusable profiles.
Contamination, where foreign DNA is inadvertently introduced, is another significant concern. Additionally, samples may contain DNA from multiple individuals, creating complex mixed profiles. These limitations mean that while DNA from fingerprints is a powerful tool, obtaining a full, clear DNA profile is not always guaranteed.