Some types of DNA tests examine one sample and can tell us something useful about it (for example deep ancestry or health). Other types of tests compare two or more samples, to see their similarities and differences, or to determine how the people who provided the samples are related. If two samples have a certain number of DNA markers in common, the people involved may be related. This is the basis for genealogical genetic testing. This is also the basis for forensic use of genetic testing. Typically in forensic DNA tests, a DNA sample found at the scene of a crime is compared to that of a suspect. If they match, the suspect (or his or her twin) was at the scene.
Identity DNA testing is similar, but in this case the two samples being compared are typically from the same person, at two different times. This could be considered a type of relationship test, except that the putative relationship is that of identity.
For example, suppose someone works in an extremely dangerous occupation. In this case, they might want to store a sample of DNA against the day when their remains might have to identified using DNA.
Identity testing can be used alongside DNA banking, where a DNA sample is stored in a secure location, and has passed through a chain of custody that will withstand scrutiny in court. One possible use for this service is to protect someone from fraudulent claims on their estate by people pretending to relatives.
