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 analysed to ascertain 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 to help diagnose disease, or used to ascertain a person's predisposition to develop a particular disease. In forensic settings, DNA typing analyses 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 person, or from a relative of the person. This can be applied in "identity" and "paternity" testing, and may be used in civil or criminal proceedings.
DNA typing can be used to uniquely 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 bloodstain, 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 heat, cold, or drying. It is ideal for identity and parentage testing because an individual’s DNA does not change during their lifetime and is the same for all the cells in the body. Except for identical twins, DNA genotype 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. DNA typing 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 the "chain of custody", and testing.
Identity testing compares DNA sequences from separate sources to ascertain if they are identical. The DNA found at a crime scene or on a victim is tested and compared to that of a suspect or to that of other suspects or convicted individuals held on a database. Testing can thus be used to match a suspect to a specific crime, link a suspect to several crimes (serial crime), link crimes with no suspect in common, or exonerate someone who has been falsely accused. DNA testing on blood, saliva, tissue, hair, or bone may also be used to ascertain the identity of individuals if no other information is available, as in the case of catastrophe victims, fragmented remains, isolated body parts, and badly decomposed bodies.
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 very rare occurrence in different individuals. Because only one-tenth of a single percent of DNA differs from one person to the next, all of the locations used for forensic DNA typing vary widely between 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 with The UK National DNA database is the largest reference database in the world. If an individual is convicted of a crime, his or her genetic fingerprint (a select list of DNA polymorphisms) is put into the database. DNA evidence from unsolved crimes is also entered, together (in England and Wales) with DNA profiles from those arrested, but not convicted. Using this system, matches in DNA profiles have helped secure convictions in thousands of crimes as well as in helping to exonerate individuals who were wrongly suspected of committing a crime.
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 National DNA database and the system cannot identify a person's physical traits or genetic risk of disease based on STRs.
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 using a mathematical estimation called a "paternity" 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.
See the article "The Universe of Genetic Testing: The Basics, Identity Testing, and Parenting Testing."