READ: DNA Evidence

DNA Evidence

DNA Evidence

Historically, fingerprints were generally considered the key individual evidence in forensic investigations. In 1985 that all changed when Alec Jeffreys applied DNA Analysis techniques to crime scene evidence to identify suspects. Using this new technique, he was able to link several samples of DNA evidence to a single person, named Colin Pitchfork, from a series of sexual assaults in the UK. Had the DNA evidence not identified Pitchfork, the wrong person would have most likely been convicted as the police had already arrested a different person for the crimes! The first person convicted using DNA evidence in the United States was a rapist from Florida in 1987 named Tommy Lee Andrews. In the next few years, "The South Side Strangler" and "The Green River Killer" were also convicted using DNA Analysis.

When entering a crime scene, investigators and officers are careful to avoid touching areas that may contain DNA evidence without gloves. To avoid contamination, they must also refrain as much as possible from releasing body fluids, such as saliva from a cough or mucus from a sneeze, while in a crime scene. DNA can be found in just about any location in a crime scene; some sources of DNA include:

1. Weapon with skin, blood, sweat or other body tissue.

2. Headwear such as a hat or mask which may contain hair, sweat, dandruff.

3. Bathroom supplies such a tissue or cotton swab which may contain body fluids such as saliva, mucus, ear wax or blood.

4. Items typically containing saliva such as toothpick, stamp, cigar/cigarette, drinking container such as a cup or bottle.

5. Bed linens which may contain hair, skin, blood, or other body fluids.

6. Fingernails which may have skin cells underneath them.

Items suspected of containing DNA evidence are packaged in paper bags and sent to the lab for further analysis. They are not packaged in plastic because plastic retains moisture which can destroy or degrade DNA evidence. Sunlight and higher temperatures can also degrade DNA samples, so investigators ensure that it is kept at room temperature and deliver it to the lab as quickly as possible.

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DNA Typing and Analysis

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.

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CODIS

Combined DNA Index System (CODIS)

 CODIS, or " Combined DNA Index System", is a collection of databases of stored DNA profiles. The profiles are obtained from evidence samples in unsolved crimes and from known individuals convicted of particular crimes. Contributions to this database are made through State crime laboratories and the data are maintained by the FBI. Using 13 standardized short tandem repeat sequences known as CODIS core loci, CODIS provides support to federal, state, and local crime labs for use in comparing DNA evidence from violent crime investigations.

Recall that the purpose of comparing the STRs, such as in the CODIS core loci is to generate a DNA Profile. DNA Profiles are the result of determining the relative positions of DNA sequences at several locations on the molecule. Each person (except identical twins) has a unique DNA profile when used in the context of the CODIS database which evaluates those 13 specific DNA locations. All 50 states have mandated the collection of DNA profiles from all offenders of certain types of crimes including sex offenders.

When comparing DNA profiles, results may be returned which result in exclusion or inclusion. Exclusion is a DNA test result indicating that an individual is excluded as the source of the DNA evidence. However, in a criminal case, "exclusion" does not necessarily equate to "innocence." Inclusion, also known as "failure to exclude" is the inability to exclude an individual as a possible source of a biological sample. This occurs when all types from a specific location in the DNA of a known individual are also present in the types for that specific location in the DNA obtained from an evidence sample.

Terms

When using CODIS, there are terms that investigators must be familiar with in order to understand the results. These terms differentiate between the various types of profiles, matches, and results that may be obtained through CODIS:

Forensic Profile - These are DNA profiles submitted to CODIS with crime scene evidence such as blood or saliva.

Convicted Offender Profile - DNA profiles submitted to CODIS of those convicted of various crimes.

Arrestee Profile - DNA profiles of people who are arrested; not all states allow this.

Missing Person Profile - DNA profiles of missing persons to use as reference if DNA evidence is found.

Unidentified Human Remains Profile - DNA profiles kept of any unidentified human remains found.

Biological Relative DNA Profile - DNA profiles of any biological relatives that are submitted to CODIS to aid in missing persons cases. These are given voluntarily and are generally used to help identify unidentified remains if needed.

Types of Hits

Forensic Hit - A CODIS match between two or more crime scene profiles. In this type of hit, samples of evidence from two or more crime scenes are found to have matching DNA profiles, but the source of the DNA is still unknown.

Offender Hit - A CODIS match between a crime scene profile and an offender profile. In this type of hit, the name of the offender is matched with an evidence sample from a crime scene.

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