Genetic Tests and DNA Typing
More recently, genetic testing has been added to the forensic pathologist's toolkit. Molecular testing of DNA from cells in a particular biological sample can be analyzed to determine the unique genetic make-up of any one person. Each individual inherits a set of genes (two copies, one from each parent) that is unique and distinctive as a fingerprint. In clinical settings, genetic testing is used most often to detect chromosomal mutations that may be present in an existing disease state or used to determine a person's predisposition for a particular disease. In forensic settings, DNA typing analyzes the genetic material from two or more sources and compares the genetic sequences to determine the likelihood that the two samples are from the same source or from a relative. This can be applied in "identity" and "parentage" testing and may be used in civil or criminal proceedings.
DNA typing can be used to ultimately identify an individual and can be done on a very small amount of sample. Often, a swab of cells from the inside of the cheek (buccal swab), a drop of blood, or a small amount of tissue can be enough to isolate a sample of DNA. Also, DNA is relatively stable and is not easily degraded by some less than ideal conditions such as heat, cold, or drying. It is ideal for identity and parentage testing because a person's DNA does not change during their lifetime and is the same for all the cells in the body. Except for identical twins, DNA is different in everyone.
In contrast to medical genetic testing, forensic DNA typing does not reveal anything about a person's health or medical history. The areas of the DNA sequence that are tested have no known ability to predict health status. This type of DNA sequencing is not the same as the in-depth, highly complex, full genomic sequencing often heard about in the news.
As with other types of forensic testing, DNA typing must follow strict protocols for proper sample collection, maintaining a chain of custody, and testing procedures. In the U.S., the Federal Bureau of Investigation's (FBI) DNA Advisory Board has implemented standards for laboratories performing forensic identity testing and AABB has developed standards for parentage testing. Both standards focus on testing and quality assurance.
Identity testing compares DNA sequences from two separate sources to determine if they are identical. The DNA found at a crime scene or on a victim is tested and compared to that of the suspected individual. Such determinations can be used to match a suspect to a specific crime, exclude someone as a suspect, link a suspect to several crimes (serial), link crimes with no suspect in common, or exonerate someone who has been falsely accused.
DNA is also used to determine identities that can't be distinguished in other ways, as in the case of catastrophe victims, fragmented remains, isolated body parts, and decomposed bodies. Labs identify individuals through analysis of DNA extracted from samples such as blood, saliva, tissue, hair, or bone.
DNA sequences consist of chemical units called nucleotides that vary in size and make up a chain-like structure. In humans, DNA sequences are over 99% similar in structure, yet the small percentage that is different makes each person unique. Labs look at these DNA sequences for matches between the presented evidence and suspects based on sequences of small segments of DNA at different locations on the person's total genetic makeup (genome). Generally, a match at thirteen sites confers confidence of identity "beyond a reasonable doubt." That's because a match at 13 sites is a rare occurrence. Because only one-tenth of a single percent of DNA differs from one person to the next, all of the locations tested for forensic DNA typing vary widely among individuals. The possibility of two people with the same DNA profile (except for identical twins) is extremely remote.
In forensic testing, obtaining DNA profile information is valuable only if there are other profiles to compare it to in order to establish a match, to exclude a profile, and/or to make an identification. In 1990, the FBI began using a computerized software program called the Combined DNA Index System or CODIS. This program is used by law enforcement officials to compare newly determined DNA profiles with existing profiles contained within the database known as the National DNA Index System (NDIS).
In criminal cases, if a suspected individual is convicted of a crime, his or her genetic fingerprint (a select list of DNA polymorphisms) is put into CODIS. DNA evidence from unsolved crimes is also entered. Using this system, matches in DNA profiles have helped in the investigation of over 100,000 crimes as well as in helping to clear some who have been wrongfully accused of crimes.
The CODIS databank is comprised of several different types of indices that are used for identification purposes. DNA profiles from individuals convicted of crimes are entered into the Convicted Offender index of the CODIS data bank. Some states have laws that require that the DNA profiles of anyone arrested be added to the Arrestee index of the CODIS system. Another index includes DNA evidence (e.g., blood, semen) from crime scenes of unsolved crimes. Other indices within this system include profiles of missing persons and relatives of missing persons that may help in the identification of found persons or in the identification of remains.
The segments of DNA used to create a "fingerprint" are called "short tandem repeats" or STRs. These STRs do not represent genes but regions that occur on stretches of DNA that lie between genes. Genetic information related to genes and inheritance patterns (e.g., medical genetic testing for disease risk) is not entered into the CODIS system and the CODIS system cannot identify a person's physical traits or genetic risk of disease based on STRs.
For more on the CODIS system, see the FBI online brochure.
DNA is also used to establish paternity or family relationships if this information is relevant to a criminal investigation or civil litigation as in seeking child support.
In cases where biological family relationships are at issue, DNA tests can either include or exclude a presumed parent, sibling, or other familial relationship that may exist through a mathematical estimation called a "parentage" or "relationship index." The probability of the relationship is estimated through a process that combines the likelihood of the genetic test results, the physical characteristics that are the expression of gene combinations (phenotype) of the parties involved (for example, eye or hair color), with the probability of other "non-genetic events," which can include information such as the location of the alleged parent at the time of conception, etc. The resulting parentage index and probability of relationship are generally admissible as evidence in court.