DNA

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  • Frequently Asked Questions
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DNA (De-oxyribo-Nucleic Acid) is the biological blueprint of living organisms, including human beings. It is the genetic material that is present in nearly every cell of the human body. Irrespective of the cell or tissue source (e.g., blood, saliva, semen, hair root), DNA from any particular individual is consistent throughout the body of that individual. It is extremely unlikely that any two individuals will have the same genetic makeup unless they are identical twins. This property of our genetic profile is utilized in identifying the source(s) of DNA in forensic investigations.

Utilizing the Polymerase Chain Reaction (PCR), forensic scientists are able to replicate target sequences of DNA known as STRs. STRs are short, tandem repeat sequences of DNA, usually 3 to 5 base pairs in length. The variation in the number of repeats at each STR location is what distinguishes one profile from another and hence, one individual from another. STRs have been widely used in medical research, in the identification of the remains of military personnel, and in the identification of victims of mass disasters such as the terrorist attack on the World Trade Center. STR analysis is widely relied upon in the criminal justice system to identify possible sources of DNA in forensic specimens. The DNA unit at the Alaska Scientific Crime Detection Laboratory has implemented the use of this DNA technology in the analysis of criminal forensic casework and offender samples for inclusion in the CODIS database.

The DNA casework unit analyzes biological samples from crime scene evidence and reference samples from known individuals. The forensic and known sample DNA profiles are compared to determine possible sources of the DNA. The analysts in the casework unit issue scientific reports on their findings and are often called upon to testify as to the findings in criminal proceedings.

Sample Storage

The DNA CODIS unit analyzes samples from qualifying arrestees and convicted offenders for inclusion in the Combined DNA Index System (CODIS). The CODIS database is the national repository for the DNA profiles from casework and offender samples from all 50 states. The profiles contained in CODIS are searched against each other at both the state and national levels. The comparison of DNA profiles between different states/laboratories generates investigative leads by identifying potential DNA matches between cases previously thought to be unrelated and between forensic crime scene evidence and known offenders. The national database is also used to assist in the identification of missing persons and unidentified human remains.

For those interested in pursuing a career in forensic DNA analysis, analysts are required to have a Bachelor’s degree in a natural science such as Biology, Chemistry or Forensic Science. In addition, they must have undergraduate credit hours and/or training in the following courses: Molecular Biology, Genetics, Biochemistry and Statistics.

I recently submitted evidence in a criminal case where the suspect in the case is also a convicted offender. Do I need to submit a known reference sample from the suspect as part of the evidence in the criminal case even though the suspect's DNA is already on file with the lab?

Samples collected under AS 44.41.035 and entered into the DNA Registration System and CODIS WILL NOT be used in place of forensic case related known (reference) samples that should be collected as part of a criminal investigation. The identification of an individual through a database match or CODIS "hit" serves as probable cause to get a warrant to obtain a known sample for subsequent use in the criminal investigation. Certified case reports are not issued as the result of database matches.

Whenever possible, known samples for use in criminal investigations should be obtained and submitted to the laboratory along with the other forensic items of evidence submitted in the case. This will expedite the DNA analysis and issuance of certified case reports for use in judicial proceedings.

What is the difference between the DNA testing performed at the AK state crime lab and mitochondrial DNA testing or Y-STR DNA testing?

There are three types of forensic DNA testing routinely performed in crime labs today. These are nuclear DNA STR (short tandem repeat) analysis, Y-STR analysis and mitochondrial DNA (mtDNA). Nuclear DNA STR analysis is the most widely used method and is the method currently utilized at the AK State Crime Lab.

Short Tandem Repeat testing targets and amplifies specific core locations in the DNA molecule. This testing can be performed to identify not only the gender of the source of the biological material but also generate a genetic profile which enables us to distinguish between two individuals with a considerable degree of confidence.

Y-STR analysis is used to amplify locations on the Y chromosome. Y chromosomes are paternally inherited and Y-STR data can only be obtained from male individuals. This type of testing may be useful when samples are mixtures of male and female DNA or when there is very little DNA from a male contributor. Y-STR testing has its limitations since close male relatives will have the same Y-STR profile.

Mitochondria are organelles in the cell that contain their own DNA. Mitochondrial DNA (mtDNA) is different from nuclear DNA. Each mitochondrion contains a few hundred to several thousand copies of mtDNA. Mitochondrial DNA is maternally inherited, meaning maternally related close relatives will have the same mtDNA profile. Therefore, mtDNA is less discriminating than nuclear DNA STR testing. However, mtDNA testing is more likely to be successful when samples are highly degraded due to age or environmental insults.

What can you tell about me from my genetic profile?

Nuclear DNA STR analysis can identify the gender of an individual. Other STR locations used for forensic purposes are non-coding regions and are not known to influence any individual traits. For example, the genetic profile generated cannot tell you if a person has blue eyes or brown, black hair or red, information about height and weight, etc.

I just served on a jury in an assault case. There was no DNA evidence linking the suspect to the victim. Doesn’t this mean that the suspect was not guilty?

No. The absence of biological evidence of a crime does not necessarily mean that a crime did not occur. Biological / DNA evidence is one piece of a larger puzzle and the totality of the evidence needs to be considered. Also, trace amounts of evidence, even if present may not always be detectable due to extremely low levels of the material.

Why does it take so long to get DNA results?

DNA analysis involves four main steps: extraction, quantification, amplification and genotyping. Once a genetic profile is obtained, the data must be interpreted and peer reviewed. Each of these steps is detail-intensive and time consuming and may take up to several hours for a single sample. Samples are often processed in batches and therefore, completing the four step process for a batch of samples can take a few weeks, once analysis is begun. In addition, due to the backlog of cases and shortage of resources, it may be several months before DNA analysis begins on a case.

I've often heard DNA testing referred to as DNA "fingerprinting". Is DNA testing replacing fingerprints?

No. There are several reasons why DNA testing should not be thought of as a replacement for fingerprints. The "DNA fingerprint" is somewhat misleading and genetic profile is a more appropriate term for the data generated. Identifying and developing a fingerprint for comparison is relatively inexpensive compared to processing a sample to obtain a DNA profile. Additionally, DNA testing has its limitations – for example, it cannot distinguish between DNA from identical twins. Fingerprints are needed to distinguish between such individuals.

I need to submit evidence to the lab for DNA analysis. What is the best way to package the evidence and what information do I need to provide?

Biological evidence should always be dried and then packaged in a porous or "breathable" container such as a paper sack or envelope. When providing the laboratory with swabs of an item, sterile swabs are preferred. All items should be clearly marked on the outside packaging with a case number, item number and a brief description of the item. Reference samples from known individuals must be labeled with the full name and one additional identifier such as the birthdate or APSIN number. Please contact the DNA section of the lab with case specific questions or for clarification.

Commercial/Private DNA Laboratories

(not an all inclusive list)

Publications

"What Every Law Enforcement Officer Should Know About Identifying DNA Evidence"

B. Budowle, T. Moretti, A. Baumstark, D. Defenbaugh, K. Keys, Population Data on the Thirteen CODIS Core Short Tandem Repeat Loci in African Americans, U.S. Caucasians, Hispanics, Bahamians, Jamaicans, Trinidadians. Journal of Forensic Science 1999;44(6): 1277-1286.

B. Budowle, A. Chidambaram, L. Strickland, C. Beheim, G. Taft, R. Chakraborty, Population studies on three Native Alaska population groups using STR loci. Forensic Science International 129 (2002) 51-57.