DNA Profiling & Fingerprinting

DNA profiling is the gold standard of forensic identification. No two people — except identical twins — share the same DNA profile, making it one of the most powerful tools available to investigators.

How DNA Profiling Works

Sample Collection

Forensic DNA analysis begins with the collection of biological evidence from a crime scene. Suitable samples include blood, saliva, semen, hair follicles (with attached root tissue), and skin cells. Even trace amounts of DNA left on surfaces through touch (known as touch DNA or low-copy-number DNA) can sometimes yield a usable profile.

DNA Extraction

Once a sample is collected, the DNA must be isolated from other cellular material. Cells are broken open (lysed) using detergents and enzymes, and the DNA is separated from proteins, lipids, and other contaminants. The result is a purified DNA sample ready for analysis.

PCR Amplification

The Polymerase Chain Reaction (PCR) is used to amplify specific regions of the DNA, creating millions of copies from even a tiny or degraded sample. PCR works through repeated thermal cycles: the DNA is heated to denature (separate) the double helix, primers anneal to the target sequences, and DNA polymerase (typically Taq polymerase, derived from the thermophilic bacterium Thermus aquaticus) extends new complementary strands. Each cycle roughly doubles the amount of target DNA.

STR Analysis

Short Tandem Repeats (STRs) are sections of DNA where a short sequence (typically 2–6 base pairs, e.g. AATG) is repeated consecutively. The number of repeats at each locus varies between individuals — one person might have 12 repeats at a given locus while another has 18. Modern forensic DNA profiling examines 20 core STR loci, providing an extremely high power of discrimination. The probability of two unrelated individuals sharing the same STR profile across all loci is typically less than one in a billion.

Capillary Electrophoresis

After PCR amplification of the STR loci, the resulting DNA fragments are separated by size using capillary electrophoresis. Fragments are drawn through a thin capillary filled with polymer gel by an electric current — smaller fragments migrate faster. Fluorescent tags attached during PCR allow a laser detector to identify each fragment, producing an electropherogram — a graph of peaks representing alleles at each STR locus.

Profile Comparison

The final step involves comparing the STR profile obtained from crime scene evidence against reference profiles — either from a suspect or from a database. If the alleles match at every locus tested, the evidence and the reference sample are considered to have originated from the same individual (or an identical twin). Statistical analysis quantifies the significance of a match by calculating the random match probability.

CODIS Database

The Combined DNA Index System (CODIS) is the national DNA database maintained by the FBI. It enables federal, state, and local forensic laboratories to exchange and compare DNA profiles electronically. CODIS contains three main categories of profiles: convicted offender profiles, arrestee profiles, and forensic (crime scene) profiles.

As of recent reporting, CODIS contains over 21 million offender profiles and has aided in more than 600,000 investigations. In 2017, the FBI expanded the core STR loci used in CODIS from 13 to 20, increasing the discriminatory power of the system and facilitating international data sharing.

Fingerprint Analysis

Fingerprint analysis is one of the oldest and most widely used methods of forensic identification. Every person's fingerprints are unique and remain unchanged throughout their lifetime (barring significant injury). Fingerprints are formed during fetal development and are influenced by random factors such as the position of the fetus and the flow of amniotic fluid.

Pattern Types

Fingerprint patterns are classified into three main categories: loops (approximately 65% of the population), whorls (approximately 30%), and arches (approximately 5%). Loops are further divided into ulnar loops (opening toward the little finger) and radial loops (opening toward the thumb). Whorls include plain whorls, central pocket loops, double loops, and accidental whorls.

Minutiae Points

While the overall pattern type provides a general classification, forensic examiners rely on minutiae — the fine details of ridge structure — for positive identification. Key minutiae features include ridge endings (where a ridge stops), bifurcations (where a ridge splits into two), and dots (very short ridge segments). A sufficient number of matching minutiae between two prints establishes identity.

Types of Fingerprints at Crime Scenes

Fingerprints found at crime scenes fall into three categories: latent prints are invisible to the naked eye and require development using powders, chemical reagents (such as ninhydrin or cyanoacrylate fuming), or alternative light sources; patent prints are visible impressions left in a substance such as ink, blood, or grease; and plastic prints are three-dimensional impressions left in soft materials like wax, putty, or wet paint.

AFIS

The Automated Fingerprint Identification System (AFIS) is a digital database that stores fingerprint records and uses algorithms to search for potential matches. When a latent print is recovered from a crime scene, it can be scanned and compared against millions of records in the database within minutes. AFIS returns a list of candidate matches ranked by similarity, which a trained examiner then reviews to confirm or exclude a match.

“Every person's fingerprints are unique. Even identical twins, who share the same DNA, have different fingerprints because they are influenced by random factors during fetal development.”

— Federal Bureau of Investigation, Fingerprint Sourcebook (2011)

DNA Profiling vs. Fingerprinting