despp: Forensic Biology
Forensic Science Laboratory

Forensic Biology


{Forensic Biology}

The Forensic Biology Section of the Laboratory is composed of six Forensic Science Examiners with over 117 years of total combined forensic experience.  These scientists methodically examine evidence, oftentimes collected in sexual assault or homicide cases, to identify the presence of blood and other body fluids, such as semen, saliva, urine and fecal material. They determine which identified samples will be forwarded to the DNA Unit for further analysis. 

The Section receives approximately 780 cases per year, approximately 500 of which include a 'CT 100' Sexual Assault Evidence Collection Kit.  Other types of evidence examined include condoms, clothing, bedding and weapons to an occasional piece of plastic fruit or portion of aluminum siding. 

Forensic science is based upon Locard’s Principle of Exchange Theory.  This theory states that when two entities come in contact, an exchange of matter will occur.  Applying this principle, the Forensic Biologists carefully examine the evidence aided by an alternate light source, if necessary.  They document forensically important information, such as damage to the evidence as well as stains and stain patterns located on the evidence.  They also collect trace materials for future examination.  Biochemical tests and microscopical examinations are used to determine if the stains in question contain blood or another type of body fluid.  Also, “touch DNA” samples may be collected from evidence, if necessary.  If the evidence requires multiple types of examinations, for example, blood plus fingerprints or firearms examination, the Forensic Biology section will typically process the evidence first and then forward the evidence to the other laboratory units.

Identification of Blood:

{Forensic Biology}

A series of tests is used to determine the presence of human blood on evidence.  After possible bloodstains are located, they are analyzed using a chemical screening test such as Kastle-Meyer (phenolthalein) reagent.  This particular test employs the heme component of the blood’s hemoglobin, to produce a positive color change reaction from clear to pink.

Since a positive screening test is only a “presumptive” test, a confirmatory test is necessary to determine the presence of human blood.  If these tests produce positive results, it is determined that the sample tested contains blood of human origin.  If these tests produce negative results, an animal bloodstain may be suspected.  A crystal test may be performed on the suspected animal bloodstain to confirm the presence of blood.  If the case warrants, additional tests may be employed to determine what species of animal blood is present.

Identification of Semen:

Stains that are suspected to contain semen also undergo a series of tests.  First, a stain is sampled and screened for the presence of acid phosphatase, an enzyme that is present in very high levels in semen.  If the application of a series of chemicals to a sample of the stain produces a purple reaction, this “presumptive” test is positive and further confirmatory tests are warranted.

The presence of semen can be confirmed by identifying spermatozoa in a stain extract.  The extract is placed on a glass slide, chemically stained and examined under a microscope.  The staining process enables the Forensic Biologist to visualize and identify spermatozoa apart from the other cells and bacteria present on the slide.  If no spermatozoa are identified, other indicators of human semen can be employed.  These tests detect the presence of semenogelin or human seminal fluid protein in extracts prepared from the stain in question.

Other Biological Material:

The Forensic Biology section is also responsible for the identification of other types of biological material such as saliva, urine and fecal material.  The identification of these materials may be used to corroborate information developed during an investigation. 

If the examiner determines that no biological fluid is present on an item of evidence, but an individual may have come in contact with that item, then a “touch DNA” sample may be collected.  Using the case information provided, along with her training and experience, the Forensic Biologist may collect a sample from an area on the evidence that is a potential site of transferred skin cells.  “Touch DNA” samples and other forensically important biological samples may be forwarded to the DNA Unit for further analysis.





Content Last Modified on 4/28/2014 11:07:34 AM