READ: DNA Evidence

DNA Typing and Analysis

Once a sample of suspected DNA evidence reaches the lab, it officially begins the process of DNA Analysis. DNA Analysis is the process of testing to identify DNA patterns or types. In the forensic setting, this testing is used to exclude or include individuals as possible sources of body fluid stains ( such as blood or saliva) and other biological evidence (bones, teeth, hair). This testing can also be used to indicate parentage. Recall that humans share mostly identical genomes; however, there are regions or loci of variation among individuals. One example of variation in humans are non-coding regions with short repeating sequences consisting of 3 to 4 nucleotides that repeat between 5 and 16 times. These loci are appropriately named short tandem repeats (STR). The Federal Bureau of Investigation (FBI) utilizes 13 STRs to identify individuals. This is because in humans, there are 13 regions of DNA, called loci, which vary from person to person and make it possible to individually identify a person. Consider the rule of probability: 2 people may share two or three sequences in the same area of the 13 loci, but the probability that 2 people would have the same sequence in all 13 loci is extremely unlikely. In fact, one statistical analysis show the probability of 2 unrelated Caucasians having identical STR profiles is about 1 in 575 trillion. The only people that have the possibility of identical DNA sequences are identical twins. The uniqueness of DNA makes it a reliable source of individual evidence.

DNA Analysis Interactivity

 A DNA profile is also sometimes called a DNA fingerprint. No two people have the same assortment of DNA sequence fragment lengths, so this is why a DNA Profile is compared to, and commonly referred to as a type of "fingerprint". It is a powerful type of individual evidence capable of discerning between individuals to either include or exclude from a given sample. Only a small amount of DNA is necessary to obtain a DNA profile. In fact, as little as one nanogram of DNA can be used for analysis. Even if DNA has been degraded, it may still be possible to obtain enough DNA to make a positive identification. Whereas the STR method is commonly used today in Forensic DNA analysis, it is not the only method of analysis used. Another older method of DNA analysis is Restriction Fragment Length Polymorphism, or RFLP.

Specialized DNA Analysis Techniques

Certain circumstances call for specialized DNA Analysis techniques. These techniques are not performed routinely, but are available when they are needed: 

Single Nucleotide Polymorphism, or SNP-SNP - is a type of analysis that involves analyzing the DNA for areas where one nucleotide is changed or mutated in a sequence. This pattern can be used to identify a person and it is often used in genetic testing to detect inheritance or susceptibility to certain diseases. It is also used in Forensics, however, because the DNA can be analyzed for an identifying pattern in which a specific single nucleotide is replaced in the sequence. It is used in forensics most often with degraded samples of DNA which were unable to be analyzed by other methods. Some of the victims of the September 11, 2001 terrorist attacks in the United States were identified using SNP analysis.

Amelogenin Gene Analysis - At times it is necessary to determine the sex of a person using their DNA sample, such as when a body is badly decomposed or involved in an explosion or arson. This can be achieved in conjunction with STR analysis and PCR by using primers that amplify the amelogenin gene. The gene is present on the X and Y sex chromosomes, and is used in DNA identification testing to determine the gender of the donor of the DNA in a biological sample. The results are determined by the number of bands found on the amelogenin gene. Because females have 2 X chromosomes, they will display only one band for the amelogenin gene while males with an X and a Y chromosome will exhibit 2 bands.

Mitochondrial DNA Analysis - Mitochondrial DNA, or mtDNA, is the DNA found in the many mitochondria in each cell of a body rather than the nuclear DNA used in other DNA analyses. mtDNA is inherited from the mother, so the person tested, their mother, any siblings from the same mother and all maternal relatives will share mtDNA. This type of DNA therefore does not identify a particular person because of the shared mtDNA with maternal relatives. Despite this limitation, it can be an important indicator in cases with limited DNA sources such as missing persons or mass disasters. It can also be used when the DNA is degraded, limited to very small quantities or damaged in some way. Common sources of mtDNA include: Charred remains, hair shafts, older skeletal remains and fingernails, other degraded samples.

Georgia Virtual, DNA Analysis, CC BY-NC-SA 3.0