What is Genetic Testing?. Genetic testing involves the examination of a person's DNA taken from cells in a sample of blood or from other body fluids/tissues for health or medical identification purposes. What is the Purpose of Genetic Testing?.
Genetic testing involves the examination of a person's DNA taken from cells in a sample of blood or from other body fluids/tissues for health or medical identification purposes.
In criminal cases it generally involves obtaining samples from crime scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions.
In order to perform genetic testing, evidence is collected from a crime scene or individual, including bodily fluids, human hair or skin. The DNA material is extracted from the biological material and then measured to see how much evidence there is to work with.
Areas of the DNA are replicated for further testing. The different regions of the DNA are separated for testing, and the DNA sequencing is preformed. The repeats in a sequence of DNA are counted, a process called genotyping, andthe information is entered into a DNA database to look for matches. The DNA evidenceiswritten upby the forensictechnician to use as evidence.
To identify individuals, forensic scientists scan 13 DNA regions that vary from person to person and use the data to create a DNA profile of that individual (sometimes called a DNA fingerprint). There is an extremely small chance that another person has the same DNA profile for a particular set of 13 regions.
If the patterns match, the suspect may have contributed the evidence sample. While there is a chance that someone else has the same DNA profile for a particular probe set, the odds are extremely slim. The question is, how small do the odds have to be when the conviction of a guilty party lies in the balance? Experts point out that using DNA forensic technology is far superior to eyewitness accounts, where the odds for correct identification are about 50-50.
Restriction Fragment Length Polymorphism (RFLP)
RFLP is a technique used to compare DNA from two samples. Special enzymes that cut the DNA in specific locations are used to digest strands of DNA. Mutations within the DNA result in strands of different lengths. RFLP is used as part of DNA fingerprinting, to detect genetic diseases and to determine genetic relationships between species.
RFLP was one of the first technologies of DNA analysis to assist with forensic investigation. With the development of more efficient DNA-analysis techniques, RFLP is not used as much anymore because it requires moderately large amounts of DNA, at least the size of a quarter. Also, samples degraded by environmental factors, such as dirt or mold, do not work well with RFLP.
Polymerase Chain Reaction (PCR) Polymerase chain reaction (PCR) is used to make millions of exact copies of DNA from a biological sample. DNA amplification with PCR allows DNA analysis on biological samples as small as a few skin cells. The ability of PCR to amplify such tiny quantities of DNA allows even highly degraded samples to be analyzed. Great care, however, must be taken to prevent contamination with other biological materials during the identifying, collecting, and preserving of a sample.
Short tandem repeat technology is used to analyzecertain areas (loci) within the DNA strand for coding and identification. This analysis in STR regions can be used to distinguish one DNA profile from another. The FBI uses a set of 13 specific STR regions for CODIS. CODIS is a software program that operates local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and missing persons. The odds that two individuals will have the same 13-loci DNA profile is about one in a billion.
Mitochondrial DNA analysis (mtDNA) is used to examine the DNA from samples that is too old or fragmentary to be analyzed by RFLP or STR. Nuclear DNA must be extracted from samples for use in RFLP, PCR, and STR. MtDNA analysis uses DNA extracted from another cellular organelle called a mitochondrion. MtDNA tests biological samples that lack nucleated cellular material, such as hair, bones, and teeth.
In the investigation of cases that have gone unsolved for many years, mtDNA is extremely valuable. All mothers have the same mitochondrial DNA as their offspring. This is because the mitochondria of each new embryo comes from the mother's egg cell. Comparing the mtDNA profile of unidentified remains with the profile of a maternal relative can be an important technique in missing-person investigations.
The Y chromosome is passed directly from father to son, so analysis of genetic markers on the Y chromosome is especially useful for determining relationships among males or for analyzing biological evidence involving multiple male contributors.
A non-profit organization called the Innocence Project is dedicated to releasing wrongfully convicted individuals through DNA testing. DNA testing has been a major factor in changing the criminal justice system. It has provided scientific proof that our system convicts and sentences innocent people to jail. Since the organization was founded, more than 250 people have been released through DNA testing in the United States, including 17 who were at one time sentenced to death.
The primary social concern with keeping and testing DNA is privacy. DNA profiles are different from fingerprints, which are useful only in identification. DNA can provide insights into many personal aspects of people and their families. People may feel angry, depressed, anxious, or guilty about their results. In some cases, genetic testing creates tension within a family because the results can reveal information about other family members in addition to the person who is tested. The possibility of genetic testing in employment or insurance is also a concern. Some individuals avoid genetic testing out of fear that it will affect their ability to purchase insurance or find a job.
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