This page provides an overview of Forensic Testing as well as Parentage DNA testing. For additional information about human genetics and DNA, visit our educational resources page.
Forensic Testing Methods
Forensic DNA testing serves a number of useful purposes. It can be used to track down criminal suspects who have left behind biological evidence, exonerate individuals who have been falsely accused of committing crimes, identify individuals who have fallen victim to violent crimes or disasters, and connect crimes that share biological evidence. It can also be used to identify the father of a child conceived through rape or incest.
Law enforcement officers and attorneys rely on a variety of forensic testing services to help them solve crimes and prosecute or defend criminal suspects. Some tests, for example, are able to detect the presence of blood, semen, or other biological substances in evidence left at crime scenes. If biological material is found, other forensic tests can be performed to determine whether the samples contain viable DNA.
Once viable DNA is detected, it can be analyzed using one of several DNA testing methods, such as STR typing, Y-chromosome STR (Y-STR) typing, or mitochondrial DNA (mtDNA) sequence analysis. The testing method used depends on the type of sample available, as well as its condition.
STR typing can be used for most forensic samples. STRs, or short tandem repeats, are DNA markers that are highly variable among individuals and therefore quite useful for human identification purposes. In addition to their variability, their small size and low rate of mutation make STRs desirable for forensic analysis. In 1998, in fact, the Federal Bureau of Investigation’s Combined DNA Index System (or CODIS, for short) established the use of 13 core STRs as an international standard for human identification testing.
Y-STR typing is useful for testing samples that contain a mixture of male and female DNA. This is because Y-STRs are found on the Y-chromosome, which only men have. Commonly used in sexual assault investigations, Y-STR typing is able to isolate a male assailant’s Y-STRs, which are variable among unrelated males and therefore useful in discriminating between male suspects.
Unlike the STR and Y-STR typing methods mentioned above, mtDNA sequence analysis is performed on mitochondrial DNA, which exists in the mitochondria of the cells, not in the nucleus. (STRs are found on the chromosomes of nuclear DNA.) Mitochondrial DNA is useful in forensic analysis because, like STRs, it is highly variable among individuals. Moreover, it exists in high quantities. This means that when a piece of evidence is too old or degraded to contain viable nuclear DNA, it may still contain viable mtDNA that can be used for human identification purposes. Hair, teeth, and bone samples are commonly used in mtDNA sequence analysis.
Once a DNA profile has been obtained from a forensic sample, it can be uploaded into CODIS. Developed by the FBI, CODIS is a national database containing DNA profiles generated at the national, state, and local levels. It allows laboratories to exchange and compare genetic profiles, which, in turn, enables crimes to be linked to convicted offenders and to other crimes.
CODIS contains two indexes. The Forensic Index contains DNA profiles generated from biological evidence left at crime scenes. The Offender Index contains DNA profiles generated from criminals convicted of sex offenses and other violent crimes. Profiles can be compared within each index and between the two indexes. When the database makes a match, the DNA laboratories that generated the associated profiles work together to confirm or deny that a true match has been made. In this way, investigations can be assisted and crimes can be solved.
Parentage DNA Testing Defined
Parentage testing is the examination and comparison of an alleged parent’s and a child’s genetic profiles to determine whether the individuals are biologically related as parent and child. There are two types of parentage tests: paternity tests and maternity tests.
Parentage testing is based on the fact that a child inherits half of his or her DNA from each biological parent. At conception, the biological mother contributes one-half of her genetic profile to the zygote through the egg cell, while the biological father contributes one-half of his genetic profile to the zygote through the sperm cell. Because DNA is inherited in this way, a child’s genetic profile can be compared to an alleged parent’s genetic profile to determine whether the two are biologically related. The profile of a biologically related child will show predictable patterns of the genetic inheritance.
The following is a primer on how parentage testing works.
Sample Collection and DNA Extraction
To begin a parentage test, DNA is collected from the tested parties. The most common method of obtaining DNA samples is the painless buccal swabbing method, but DNA can also be obtained from blood or tissue samples and, many times, from ordinary items such as chewing gum, cigarette butts, and clothing stained with blood or semen.
In a paternity test, DNA samples must be collected from the alleged father and the child; the child’s mother may also be collected if she is available. In a maternity test, samples must be collected from the alleged mother and the child; if available, the child’s father may be tested as well.
Once a sample is collected from a tested party, the DNA is extracted from it, purified, and replicated. The replicated DNA then undergoes capillary electrophoresis, which produces a DNA profile.
Comparison of DNA Profiles
A person’s DNA profile is made up of a number of genetic markers, which are specific sequences of DNA at particular locations that scientists have determined to be useful as identifying marks for individuals. Every person has two copies of these markers; one set is inherited from the mother, and the other set is inherited from the father.
A DNA profile reveals the allele sizes of the different markers, and in a parentage test, these are what the DNA analyst compares to determine whether the alleged parent and child are biologically related. If the tested parent and child are biologically related, their profiles will have at least a 50% similarity.
When the allele sizes of the DNA markers at a specific locus match between an alleged parent and a child, the marker is given a Paternity Index (or Maternity Index) value. This value depends on the frequency with which the marker appears in the population. When the allele sizes of the DNA markers at a specific locus do not match between an alleged parent and a child, the marker is given a Paternity/Maternity Index of 0.
After all the markers have been compared, the Paternity/Maternity Indexes are multiplied. This produces the Combined Paternity/Maternity Index, which is converted into a Probability of Paternity/Maternity percentage. The Probability of Paternity/Maternity percentage states the probability that the tested parent is the biological parent of the child. For an inclusion result, the Probability of Paternity/Maternity is 99.9% or higher.
As an industry standard, if an alleged parent’s profile shows three or more DNA markers that do not match the child’s markers, then that person is excluded as the biological parent of the child. (The Probability of Paternity/Maternity is 0%.)
Other Family Relationship Tests
In addition to parentage tests, companies in the parentage testing industry usually perform other family relationship tests that operate under the same basic principles as paternity and maternity tests. Examples of these other family relationship tests include grandparentage tests and siblingship studies.