How It’s Done

Sources of DNA Evidence

The biological material used to determine a DNA profile include blood, semen, saliva, urine, feces, hair, teeth, bone, tissue and cells.

Samples that May be Used

Investigators collect items that could have been touched or worn by persons involved in a crime. The following items may contain DNA material:

The best evidence occurs when a person’s DNA is found where it is not supposed to be. For example, consider a breaking-and-entering that occurred in a residential area. Near the point of forced entry, a knit cap was found which the homeowners confirm was not theirs. Several head hairs were recovered from the inside, one of which had a root with tissue attached, which made it possible to obtain a DNA profile. The DNA profile was used to identify the perpetrator.

A crime scene investigator uses a swab to collect blood from a crime scene. (Courtesy of NFSTC)

An evidence photo of a ciggarette on the floor, framed by measuring tapes.

A cigarette butt found at a crime scene may contain valuable DNA material in the dried saliva. (Courtesy of NFSTC)

An evidence photo of a hammer on the floor, framed by measuring tapes and surrounded by blood drops.

DNA evidence from both the victim’s blood and the perpetrator’s skin cells may be available from this hammer. (Courtesy of NFSTC)

As technology advances, forensic scientists are able to analyze smaller and smaller biological samples to develop a DNA profile. For example, if a person touched an object or weapon, skin cells may have been left behind. This low-level DNA is sometimes referred to as “touch DNA”. It can even be collected from a victim’s skin or bruises where they were handled roughly. Low-level DNA samples may be helpful when examining evidence where it would be difficult to retrieve fingerprints—such as textured surfaces on gun handles or automobile dashboards. However, not all jurisdictions have the capability to process this evidence.

To compare the victim’s or suspect’s DNA profile to the recovered crime-scene DNA, the laboratory will need to have their known biological samples available for a side-by-side comparison. These known samples are called reference samples. In some jurisdictions, a DNA sample is routinely taken from an arrestee during the process of booking and fingerprinting. However, this is an evolving area of law and states vary in their laws governing the collection of DNA from arrestees. Sometimes a court order is required to retrieve a reference from a person of interest. Reference samples are always collected from victims unless they choose not to cooperate with the investigation; in that case, a court order might be required.

A woman is having the inside of her cheek swabbed with a collection brush.

Reference samples are often collected by swabbing the inside of the cheek.

In addition to unknown and reference samples, elimination samples are often collected from consensual sex partners and others, such as first responders, crime scene personnel and analysts working the case so they can be excluded from the investigation.

It is important that biological evidence be properly collected and preserved as it can easily degrade when exposed to heat or humidity. Storing evidence in cool environments is preferred; however, research has shown that room temperature conditions are suitable for storing dried stains as long as the humidity is controlled. Liquid samples should be transported in refrigerated or insulated containers.

Who Conducts DNA Analysis

DNA analysts working in laboratories that participate in the FBI’s National DNA Index System (NDIS) and/or are accredited by a recognized organization must meet specific educational and training requirements. At a minimum, a bachelor’s degree in biology, chemistry, or a forensic science-related area is required. In addition, the analyst should have successfully completed nine hours of coursework at the undergraduate or graduate level covering the following subject areas: biochemistry, genetics, molecular biology, as well as coursework or training in statistics and/or population genetics, as it applies to forensic DNA analysis.

To ensure analysts’ skills are kept up to date, analysts who are actively employed at a crime laboratory are also required to meet continuing education requirements. These requirements are stipulated by the FBI’s Quality Assurance Standards (QAS).

The specialists who conduct DNA analysis in the laboratory are referred to by several different titles, including: Crime Laboratory Analyst, Forensic Examiner, Forensic Scientist and Forensic Laboratory Analyst.

How and Where DNA Testing is Performed

DNA testing must be conducted in a laboratory with dedicated facilities and equipment that meet the FBI’s stringent QAS requirements. Most publicly funded DNA crime laboratories in the United States are part of state, regional or municipal law enforcement agencies and accept submissions from multiple agencies.

Prior to performing DNA analysis at the laboratory, initial testing is often conducted at the crime scene to determine the type of biological material in question. Screening for the presence of biological materials may also be conducted in the laboratory to determine if a specific biological fluid may be present. Most biological screening tests are presumptive in nature and do not specifically identify a bodily fluid.

To determine who deposited biological material at a crime scene, unknown samples are collected and then compared to known samples taken directly from a suspect or victim.

Most DNA samples submitted to a laboratory undergo the following process:

  1. Extraction is the process of releasing the DNA from the cell.
  2. Quantitation is the process of determining how much DNA you have.
  3. Amplification is the process of producing multiple copies of the DNA in order to characterize it.
  4. Separation is the process of separating amplified DNA product to permit subsequent identification.
  5. Analysis & Interpretation is the process of quantitatively and qualitatively comparing DNA evidence samples to known DNA profiles.
  6. Quality Assurance is the process of reviewing analyst reports for technical accuracy.

A gloved hand holds a small test tube containing DNA over a centrifuge

During extraction, a centrifuge is used to concentrate the sample to the base of the tube.

How the Results are Interpreted

The DNA analysis process provides the analyst with a chart called an electropherogram, which displays the genetic material present at each loci tested (each of the gray bars on the graph below, except for the last one, correspond to a locus; the final gray area is used to indicate the gender of the individual). In a complete profile, each person will exhibit either one or two peaks (alleles) at each locus. The following electropherogram is an example of a profile from a single individual (i.e., a “single-source” profile):

graph displaying a series of jagged lines with tall peaks in particular places

Click image to view larger.

Loci that display only one allele indicate that the individual inherited the same marker from both parents at this locus. Where two alleles are displayed, the individual inherited different markers.

2 graphs displaying lines with matching peaks

Click image to view larger.

This image shows that the first four loci from the unknown evidence sample collected at the scene match the sample collected from the suspect. (This process would be repeated for all 13 loci.)

Note: The height of each peak must exceed a predetermined quantity threshold to be used in the analysis.

Is the suspect included?

In practice, evidence often contains a mixture of DNA from more than one person. These mixtures can be very challenging to analyze and interpret. In the following example, each marker from the suspect sample is included in the mixture profile collected from the evidence.

2 graphs displaying lines with matching peaks. the top line includes additional peaks.

Click image to view larger.

Partial Profiles:

If any locus is missing an allele, this is considered a partial profile. Partial profiles can happen for a variety of reasons, such as when a sample is degraded. If a sample has peaks at every locus, but any of them fall below a predetermined threshold, this would also be considered a partial profile.

a short graph which display no peaks past the mid point from left to right.

The partial DNA profile displayed above is missing peaks at two loci. Click image to view larger.

Comparing Profiles Against a Central Database

To enable profiles to be searched against a large, national database, the FBI created the National DNA Index System (NDIS) in 1998. This national database is part of the Combined DNA Index System (CODIS) that enables law enforcement agencies throughout the nation to share and compare DNA profiles to help investigate cases. As of 2012, there are more than 10 million DNA profiles in the system and CODIS has produced leads that have assisted in almost 170,000 investigations.

Once a laboratory enters a case into CODIS, a weekly search is conducted of the DNA profiles in NDIS, and resulting matches are automatically returned to the laboratory that originally submitted the DNA profile. CODIS has three levels of operation:

information in this image duplicated from previous paragraph.

The NDIS databases contain DNA profiles from:

Each database has its own rules regarding the number of STR markers that must be present for the profile to be uploaded. The National DNA Index System (NDIS) requires that 13 autosomal STR markers be tested (with more loci expected in the future), and the profile must contain information for at least 10 loci. The requirements are less stringent for state and local databases. States require the profile to have information for seven or more loci, and the local database requires at least four loci to be present to be uploaded.

Back to top of page ▲