DNA TIPS Technical Difficulties

Y-Chromosome Analysis

Several genetic markers have been identified on the Y chromosome that can be used in forensic applications. Y-chromosome markers target only the male fraction of a biological sample. Therefore, this technique can be very valuable if the laboratory detects complex mixtures (multiple male contributors) within a biological evidence sample. Because the Y chromosome is transmitted directly from a father to all of his sons, it can also be used to trace family relationships among males. Advancements in Y-chromosome testing may eventually eliminate the need for laboratories to extract and separate semen and vaginal cells (for example, from a vaginal swab of a rape kit) prior to analysis.

STR Analysis

Short tandem repeat (STR) technology is a forensic analysis that evaluates specific regions (loci) that are found on nuclear DNA. The variable (polymorphic) nature of the STR regions that are analyzed for forensic testing intensifies the discrimination between one DNA profile and another. For example, the likelihood that any two individuals (except identical twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion or greater.

Inclusions

When the results obtained from the standard sample from a known individual are all consistent with or are all present in the results from the unknown crime scene sample, then the results are considered an inclusion or nonexclusion. The term "match" is also commonly used when the test results are consistent with the results from a known individual. That individual is included (cannot be excluded) as a possible source of the DNA found in the sample. Often, statistical frequencies regarding the rarity of the particular set of genetic information observed in the unknown evidence sample and for a known individual are provided for various population groups.

Questioned or Unknown Samples
Samples From Unidentified Bodies
Reference Samples From Known Individuals
Samples to Use When No Conventional Reference Samples Are Available
Reference Samples From Individuals Who Have Been Transfused
Use of Samples From Relatives for Testing
Determination of Paternity or Maternity of a Child or Fetus

Steps in DNA Sample Processing

Following is a a review of the steps involved in processing forensic DNA samples with STR markers. STRs are a smaller version of the VNTR sequences first described by Dr. Jeffreys. Samples obtained from crime scenes or paternity investigations are subjected to defined processes involving biology, technology, and genetics.

Analyzing DNA Evidence

Several basic steps are performed during DNA testing regardless of the type of test being done. The general procedure includes: 1) the isolation of the DNA from an evidence sample containing DNA of unknown origin, and generally at a later time, the isolation of DNA from a sample (e.g., blood) from a known individual; 2) the processing of the DNA so that test results may be obtained; 3) the determination of the DNA test results (or types), from specific regions of the DNA; and 4) the comparison and interpretation of the test results from the unknown and known samples to determine whether the known individual is not the source of the DNA or is included as a possible source of the DNA.

Any probative biological sample that has been stored dry or frozen, regardless of age, may be considered for DNA analysis.

Chain of Custody

The chain of custody of evidence is a record of individuals who have had physical possession of the evidence. Documentation is critical to maintaining the integrity of the chain of custody. Maintaining the chain of custody is vital for any type of evidence. In addition, if laboratory analysis reveals that DNA evidence was contaminated, it may be necessary to identify persons who have handled that evidence.

In processing the evidence, the fewer people handling the evidence, the better. There is less chance of contamination and a shorter chain of custody for court admissibility hearings.

Contamination. Because extremely small samples of DNA can be used as evidence, greater attention to contamination issues is necessary when identifying, collecting, and preserving DNA evidence. DNA evidence can be contaminated when DNA from another source gets mixed with DNA relevant to the case.

Crime Scene Integrity

Protection of the crime scene is essential to the protection of evidence. Safeguarding and preserving evidence is fundamental to the successful solution of a crime. Remember, while documenting evidence at the crime scene, to include descriptions of whether evidence was found wet or dry. An example of this documentation would include blood spatters.

Basic Biology of DNA

DNA is the abbreviation for deoxyribonucleic acid, which is the genetic material present in the cells of all living organisms. DNA is the fundamental building block for an individual's entire genetic makeup. A person's DNA is the same in every cell (with a nucleus). The DNA in a person's blood is the same as the DNA in their skin cells, semen, and saliva.

History of Forensic DNA Analysis

DNA typing, since it was introduced in the mid-1980s, has revolutionized forensic science and the ability of law enforcement to match perpetrators with crime scenes. Thousands of cases have been closed and innocent suspects freed with guilty ones punished because of the power of a silent biological witness at the crime scene.

'DNA fingerprinting' or DNA typing (profiling) as it is now known, was first described in 1985 by an English geneticist named Alec Jeffreys. Dr. Jeffreys found that certain regions of DNA contained DNA sequences that were repeated over and over again next to each other. He also discovered that the number of repeated sections present in a sample could differ from individual to individual. By developing a technique to examine the length variation of these DNA repeat sequences, Dr. Jeffreys created the ability to perform human identity tests.

Basics of DNA Typing

Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products.

In criminal cases, this 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 (markers).

If the sample profiles don't match, the person did not contribute the DNA at the crime scene.

If the patterns match, the suspect may have contributed the evidence sample.

DNA from crime scenes also can be compared to profiles stored in a database.

DNA Database Hits

DNA database hits were very rare events until the late 1990s. A combination of the maturity of CODIS software, adoption of STR technology, and availability of funding has resulted in a significant increase in the number of hits.

There have been tens of thousands of DNA database offender hits resulting from searches at the state and local levels . They included hits between states, linking unsolved forensic profiles to convicted offenders from other states or to forensic cases from other states. DNA database searches have provided breaks in numerous high profile rape and homicide cases. This demonstrates the effectiveness of such searches in providing investigative leads in unsolved cases.

Types of Profiles in the Database

Convicted offender profiles account for most entries in a state's DNA database. Every state in the has passed legislation regarding collection of DNA samples from convicted offenders.

Learn more about State DNA Statutes.

Forensic Profiles. The second most common entry in DNA databases is forensic profiles, developed from evidence in forensic cases. Significant numbers of forensic profiles entered by states into CODIS are probative profiles from cases where the perpetrator is not known, commonly referred to as unsolved cases. Additionally, states also enter forensic evidence profiles that match the reference profile of the suspect in that case (solved cases). Forensic profiles are entered into CODIS in the hope that a match will result from a search of the database, and an investigative lead generated.

NDIS Procedures and Administration

Each state is responsible for determining its own policies regarding samples allowed in the state's DNA database, primarily via legislation regarding the creation and use of DNA databases. Each state can set its own policies, data entry, and/or search schedules, and other activities on CODIS within that state. However, there is still an expectation of reasonable uniformity amongst states about the use of CODIS.

Levels of the Database

The National DNA Index System (NDIS) is a system of DNA profile records input by criminal justice agencies (including State and local law enforcement agencies).
The Combined DNA Index System (CODIS) is the automated DNA information processing and telecommunication system that supports NDIS.

Capabilities of CODIS Software

The current CODIS software is designed for the storage and searching of short tandem repeat (STR) profiles. The same version of the software is used by all participating laboratories at the local, state, and federal levels. Although the main version of CODIS is for handling STR results, a separate version exists for the entry and searching of mitochondrial DNA (mtDNA) profiles.

The four primary functions of the current CODIS software are:

DNA profile entry and management: the function dealing with the database DNA profiles.
Searching: the function allowing a search of database DNA profiles.
Match management: the function managing search results. For example, it allows a laboratory to record and distinguish whether a particular match is an offender hit or a forensic hit, and whether the match is within or outside of the state.
Statistical calculations: the function enabling laboratory personnel to calculate profile statistics, based on the laboratory's or FBI's population frequency data .

Basics of How CODIS Works

CODIS uses two indexes to generate investigative leads in crimes for which biological evidence is recovered from a crime scene. The convicted offender index contains DNA profiles of individuals convicted of certain crimes ranging from certain misdemeanors to sexual assault and murder. Each State has different "qualifying offenses" for which persons convicted of them must submit a biological sample for inclusion in the DNA database. The forensic index contains DNA profiles obtained from crime scene evidence, such as semen, saliva, or blood. CODIS uses computer software to automatically search across these indexes for a potential match.

Combined DNA Index System

CODIS stands for Combined DNA Index System. It is the core of the national DNA database, established and funded by the Federal Bureau of Investigation (FBI), and developed specifically to enable public forensic DNA laboratories to create searchable DNA databases of authorized DNA profiles. The CODIS software permits laboratories throughout the country to share and compare DNA data. In addition, it provides a central database of the DNA profiles from all user laboratories. A weekly search is conducted of the DNA profiles in this national database, known as the National DNA Index System (NDIS), and resulting matches are automatically returned by the software to the laboratory that originally submitted the DNA profile.

DNA Technology Advancements

Recent advancements in DNA technology have improved law enforcement's ability to use DNA to solve old cases. Original forensic applications of DNA analysis were developed using a technology called restriction fragment length polymorphism (RFLP). Although very old cases (more than 10 years) may not have had RFLP analysis done, this kind of DNA testing may have been attempted on more recent unsolved cases.

However, because RFLP analysis required a relatively large quantity of DNA, testing may not have been successful. Similarly, biological evidence deemed insufficient in size for testing may not have been previously submitted for testing. Also, if a biological sample was degraded by environmental factors such as dirt or mold, RFLP analysis may have been unsuccessful at yielding a result.