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A Short Tandem Repeat, or STR, is a portion of DNA in which small segments are repeated a variable number of times. For STR markers of interest in human identity testing, including forensic and family relationship testing, typically, there are 10 to 25 possible variations of a given marker, with each person having just one or two variations. By looking at a moderate number of STRs, a DNA profile is determined that is virtually unique for each individual, except in the case of identical twins. STRs are the most common genetic marker used to determine identity in forensic and paternity applications.
A DNA profile can be determined from any type of biological specimen containing nuclear DNA, including blood or a tissue sample such as a cheek swab. These specimens may be used for paternity testing, for determining profiles of suspects, victims and felons, and for determining the profile of an animal in both susceptibility and traceability applications as detailed further in the description of animal and agriculture testing below. The STR markers used to establish a person's identity have been selected specifically to be able to confirm identity without inadvertently providing other information about the individual, such as information concerning the individuals current health or susceptibility to medical conditions.
A DNA profile can also be determined from DNA contained in biological evidence from a crime scene, such as blood stains, semen, hair, skin, bone, teeth and even minute traces of saliva resident in cigarette butts or postage stamps. DNA profiles from evidence can be compared with that of a suspect or victim, and can be catalogued in a database much like fingerprints for future comparison. DNA testing can also be used to confirm that a suspect committed a particular crime or exonerate a falsely accused or convicted person. In various countries around the world, DNA samples are collected from convicted felons, profiled and entered into a national database. Evidence from crime scenes in which no suspect has yet been identified can then be analyzed and compared with this database to possibly identify a suspect. In the US, there are 13 standard STR markers that are analyzed by public and private forensic laboratories to establish DNA profiles that are submitted to the FBI-managed national felon database known as the Combined DNA Index System, or CODIS.
DNA testing may also be used in paternity and other family relationship testing. Since DNA markers are inherited, the profile of a child can be compared with that of the alleged father to confirm or exclude him as the child's biological father. Similarly, DNA markers can prove family relationships for several other purposes including individuals immigrating to a country or for children being adopted by foreign nationals. Recently, individuals and employers have used DNA testing to establish a person's genetic identity and store it for future reference in the event of an emergency or accident.
The second form of variability in DNA involves a change in a Single Nucleotide Polymorphism, or SNP. Identifying SNPs can have significant effects on both disease susceptibility and drug response. It is the current industry estimation that each individual has between three and ten million SNPs. By looking at a moderate number of SNPs, usually between 50 and 70, a unique genetic profile can be determined for an individual human, animal or organism. SNPs also have an advantage over STRs of being contained in smaller segments of DNA that are more likely to survive the environmental degradation that can occur due to extreme elements such as water and heat. As such, they may be useful in establishing a DNA profile when STR markers fail to produce a reliable result. It is this characteristic that prompted the use of SNPs to help identify victims of the World Trade Center disaster.
SNPs can also be used to determine patterns associated with disease susceptibility or resistance, such as the identification of SNPs in sheep which can be used to determine which sheep have susceptibility or resistance to the animal disease scrapie. By identifying sheep that are susceptible to scrapie, the disease may ultimately be bred out of the sheep population.
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