Getting and Analysing Forensic DNA Evidence

DNA profiling is a rather interesting process. By accessing an individual’s DNA, scientists are able to find out important information about them, ranging from their paternity and heritage to their possible involvement in a crime.

How is a person’s DNA evidence collected?

There are several sources of DNA that can be used to create a DNA profile, and these include tissues and cells from hair, teeth, bone, blood, saliva, semen, urine, and even feces.

Investigators will collect items that could have been used by a person suspected of involvement in the crime. These are often personal items such as hats, gloves, masks, or any article of clothing. If seen at the crime scene, the investigators will also collect tools, weapons, and other pieces of evidence.

Investigators will also include other random items like dirty laundry, used bedding, fingernail cuttings, cups or bottles, cigarettes, condomsstamps or envelopes, and hygiene items like toothbrush and toothpicks, tissue, eyeglasses, or any underwear for a presence of semen.

Sometimes, evidence can be found in an unexpected item. For example, in a case of burglary, the investigators would ask the victims to look around to identify items that do not belong to them. A strange cap that does not belong to the victims will be taken as evidence, and if there are hairs on its inside, it is possible to create a DNA profile, which would then be used to identify the perpetrator of the crime. Other cases may include the saliva found in a cigarette butt and the skin cells found on the handle of the tool used in committing the crime.

All these are possible due to the technological advancements made in the field of forensic science. It used to be impossible to analyse low-level DNA, which is sometimes called “touch DNA,” one example of which is the skin cells left by the perpetrator on the tool they used. Now, it is possible for forensic scientists to develop a DNA profile by analysing these smaller biological samples.

DNA evidence may even be collected from the victim’s skin or bruises as chances are if they were handled roughly, there might be some of the perpetrator’s skin cells left. These low-level DNA samples are helpful in examining evidence in cases where it would be difficult to collect fingerprints, particularly on gun handles with textured surfaces or car dashboards.

Unfortunately, not every jurisdiction is currently capable of processing evidence from low-level DNA samples.

How is forensic DNA evidence analysed?

The laboratory will need two samples for a side by side comparison: a sample from either the victim or the suspect and the recovered DNA sample from the crime scene. These known samples are called “reference samples,” which are often collected through buccal cells collected by swabbing the inside of the cheek.

The laws governing the collection of DNA from detainees vary in every state and country. Some jurisdictions routinely take a DNA sample from the person arrested through booking and fingerprinting, while in others, a court order is required before the person’s DNA sample is collected.

On the other hand, law enforcers will always collect reference samples from the victims except if they decide to not cooperate with the investigation. In the case of the latter, a court order might be obtained.

In addition to reference samples and unknown samples, the laboratory also analyses elimination samples to exclude other people whose DNA might be found in the crime scene. These people may include the victim’s consensual sex partner, first responders, crime scene investigators, and all the other people working on the case.

When collecting biological evidence, it is very crucial to collect them properly as well as preserve them to keep them from degrading when exposed to heat or humidity. It is preferable to store evidence in a cool environment, although it is also possible to store samples of dried stains in an environment with room temperature as long as the humidity is controlled. In the case of liquid samples, they should be transported in refrigerated or insulated containers.

Who is accredited to test forensic evidence?

The specialists who conduct laboratory DNA analysis can be called by several titles, such as Crime Laboratory Analyst, Forensic Examiner, Forensic Scientist, and Forensic Laboratory Analyst.

There are DNA analysts who work specifically in laboratories that participate in the FBI’s National DNA Index System or NDIS or laboratories that are accredited by a recognised organisation. These DNA analysts need to meet specific educational and training requirements, such as a bachelor’s degree in biology or chemistry or a course that is related to forensic science.

The analysts also need to complete nine hours of coursework that covers the subject areas of biochemistry, genetics, molecular biology. They may also need to complete other coursework in statistics and population genetics.

Aside from these requirements, DNA analysts need to keep their skills up to date, which is why actively employed analysts are also required to fulfill continuing education requirements as stipulated by the FBI’s Quality Assurance Standards or QAS.

How is DNA testing performed?

DNA testing, which is conducted in a laboratory with dedicated facilities that meet the FBI’s QAS requirements, usually take place in publicly funded DNA crime laboratories. These laboratories are part of the government law enforcement agencies and can accept submissions from other agencies.

Before the DNA analysis is performed at the laboratory, scientists need to conduct initial testing at the crime scene to determine the type of biological material the sample is. To determine if a specific biological fluid is present, they will need to screen for the presence of biological materials, which may also be conducted in the laboratory. These screening tests are presumptive in nature and will not specifically identify a bodily fluid.

To determine to whom the biological material belongs, unknown samples are collected, which are then compared to reference samples or known samples taken from the suspect or the victim.

The process of analysing a DNA sample begins by extracting or releasing the DNA from the cell. During this step, a centrifuge will be used to concentrate the DNA sample to the base of the tube. Next, it is quantitated, wherein the scientists determine how much DNA they have. They then amplify the sample, which involves producing multiple copies of the DNA for characterisation.

Next, the scientists will separate the amplified DNA product to allow subsequent identification. They will then analyse and interpret the DNA sample quantitatively and qualitatively, comparing the evidence samples to known DNA profiles. Finally, the reports made after testing will undergo quality assurance and will be reviewed to ensure technical accuracy.

How are DNA test results interpreted?

Understanding the results is very easy, the analyst will be able to come up with a chart called “electropherogram.” This chart displays the genetic material present at each loci tested. If the loci display only one allele, it indicates that the individual inherited the same marker from both parents at this locus. On the other hand, if two alleles are displayed, it means that the individual inherited different markers.

In some cases, the evidence contains a mixture of DNA with different people as sources, which makes it challenging to analyse and interpret the sample. There are also some cases wherein the locus misses an allele. In this case, it would be considered a partial profile. Partial profiles can occur due to a variety of reasons, such as a sample that is degraded. It can also happen when the sample has peaks at every locus but any of them falls below a predetermined threshold.

Is there a central database of DNA profiles?

One factor that has helped elevate the accuracy of DNA profiling is the presence of a central database. In the case of the United States, the National DNA Index System or NDIS was created by the FBI in 1998. This database is part of the Combined DNA Index System or CODIS, which allows different law enforcement agencies from all over the country to share and compare DNA profiles to investigate cases.

Once a laboratory enters a case into the CODIS, a weekly search is conducted among the DNA profiles in the NDIS. The resulting matches are automatically reported to the laboratory that submitted the case. As of 2012, there were more than 10 million DNA profiles in the system. The CODIS has also produced leads that have been able to assist in almost 170,000 crime investigations.

The Combined DNA Index System (CODIS) has three levels of operation, namely the Local DNA Index System or LDIS, the State DNA Index System or SDIS, and the National DNA Index System or SDIS.

The NDIS also contains several databases, and each database has its own rules regarding the number of STR markers necessary for each profile. The NDIS databases contain the DNA profiles of the following:

  • Convicted offenders or individuals convicted of crimes
  • Arrestees or profiles of arrested persons (again, the law for collecting DNA from arrestee varies from state to state)
  • Forensic unknowns or the profiles of unknown individuals found in crime scene evidence
  • DNA reference profiles from missing persons
  • DNA profiles of the biological relatives of these missing persons
  • Unidentified human remains.

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