Biology/DNA

Deoxyribonucleic acid (DNA) is the name given to the genetic material that imparts human individuality. In comparing a human cell to an egg, nuclear DNA resides in the yolk. The areas of DNA with forensic value are located in non-protein coding regions and contain short sequences of high variability from person to person. This variability contributes to the uniqueness of individual DNA profiles, and the short sequences allow greater resistance to degradation; two highly desirable features for the analysis of a forensic sample. Short tandem repeat sequences (STRs) are extremely discriminating in forensic DNA analysis and are, thus, indispensable in the testing of DNA as an analytical approach to impart human individuality via generation of a DNA profile.

The processing of biological material to obtain a DNA profile is accomplished in four automated steps. First, DNA is extracted from a forensic sample. In this step, the DNA that is confined to the nucleus of a cell is released and purified into a small volume of liquid referred to as DNA extract.  When the presence of semen is indicated in a forensic sample, a differential extraction procedure is followed whereby the sperm cells can be separated from the non-sperm cells. By this method, one sample is processed as two fractions: a ‘sperm’ fraction and a ‘non-sperm’ fraction. The second step is to quantify the extracted DNA to determine the total amount of human and male specific DNA in the sample to try and target an ideal amount of DNA for the downstream process. Additionally, this step gives insight about the quality of the DNA sample by identifying if potential degradation or inhibition exists. The third step is amplification of specific DNA sequences (STRs) by a procedure called the Polymerase Chain Reaction (PCR). In this step, an instrument known as a thermal cycler acts as a copy machine to make millions of copies of the STRs being tested for, a similar process that the human body does every day when it copies DNA to make new cells. The current number of locations amplified to produce the typical DNA profile includes 23 autosomal STR markers (found on the standard 22 chromosome pairs), 1 marker specific for gender called Amelogenin (found on the X and Y chromosomes), and 3 markers specific to the Y chromosome only for a total of 27 altogether. Finally, amplified DNA is a separated and detected by capillary electrophoresis using the 3500 Genetic Analyzer.

Once the laboratory DNA process is completed, the raw data from the genetic analyzer is analyzed utilizing GeneMapper ID-X analysis software resulting in the DNA profile that will be used for comparisons. Profiles from questioned items of evidence are evaluated first for sufficient quality to do comparisons.  If acceptable, profiles from known individuals are compared to determine if the person is a potential contributor to the unknown DNA profile. Using probabilistic genotyping software call STRmix, a likelihood ratio is calculated to relay the statistical weight of the comparison. Likelihood ratios compare two competing scenarios based on the questioned evidence profile. One scenario includes the person of interest (the reference profile being compared) as a contributor to the questioned profile and the other scenario does not include the person of interest (POI). Conclusions can result in support for inclusion, support for exclusion, or be inconclusive for either inclusion or exclusion.

Another resource available to the DNA section is Y-STR testing. This testing focuses on amplifying specific DNA markers located on the Y chromosome only which is primarily useful in cases where there is a large amount of genetically female DNA mixed with a smaller amount of genetically male DNA. In these instances, traditional autosomal STR testing may result in the male profile being overshadowed by the female profile, a common limitation in sexual assault investigations. Y-STRs, although not as discriminatory, have been invaluable to delineate a male component to a mixture.

The Combined DNA Index System (CODIS) is a DNA database administered by the FBI to allow for profiles from crime scene evidence to be searched against a database of known offenders and other crime scene profiles. There are three different levels of CODIS: Local (LDIS), State (SDIS), and National (NDIS). Local levels are independent labs that perform DNA testing for law enforcement agencies. Each state has an SDIS level that analyzes and houses lawfully owed DNA samples that are submitted from offenders/arrestee/detainees/other legal means in accordance with their state statute in addition to profiles from local labs in that state. The NDIS level is managed by the FBI and houses profiles from each SDIS that meet eligibility requirements to be uploaded.

Searches at each level can result in matches that either identify a potential perpetrator of the crime and/or link crimes to each other indicating a potential serial offender.  Resulting hits are considered investigative leads that are relayed to the investigating agency for follow up.

CODIS can also be used in cases involving missing persons. Samples from relatives of missing persons or from the missing person themselves (via an object containing that person’s DNA) can be searched against profiles from unidentified human remains to determine if a potential biological relationship exist.