Biology (DNA, Serology & Bloodstain Pattern Analysis)

Forensic Scientists assigned to the Biology Unit examine evidence for the presence of biological substances and analyze samples to obtain a DNA profile. The Biology Unit compares known profiles from victims and/or suspects to evidentiary sample profiles for inclusion or exclusion in a case. The profiles are then entered into the Combined DNA Index System (CODIS). CODIS compares evidence from one crime scene to another as well as arrestee and convicted offender profiles nationwide.  The Biology unit also conducts bloodstain pattern analysis in the lab and at crime scenes. Members of this unit provide expert witness testimony in court regarding their findings in the areas of Serology (body fluid identification), DNA, and Bloodstain Pattern Analysis.

Terminology

Commonly used terms when working with biological evidence:

  • Serology: Detection of body fluids.
  • DNA: Genetic material used to differentiate between individuals.
  • Profile: The combination of all the DNA markers used to identify an individual.
  • Loci: Location of DNA analyzed. Mesa analyzes 16 loci.
  • PCR/STRs: Polymerase Chain Reaction/Short Tandem Repeats. Type of DNA analysis conducted by our laboratory.
  • CODIS: Combined DNA Index System. DNA database used to search DNA profiles.
  • Statistics: Weight given to the DNA match/Inclusion of an individual to the crime scene evidence.
  • Buccal Swab: Swab of the inside of the mouth used to collect a known sample.
  • Known: Term used to refer to reference DNA samples taken from individuals.
  • Unknown: Term used to refer to crime scene evidence.

Examinations Performed

Body Fluid Testing - Which body fluid does the sample originate from?  Detection, classification and study of various body fluids such as blood, semen and saliva, and their relationship(s) to a crime scene.

Species Testing - Is this sample of human origin or non-human? Is a technique based on antibody testing to determine if a sample is from higher primate origin or not.

DNA Testing - Who's DNA does this sample belong to? A technique employed by forensic scientists to assist in the identification of individuals by their respective DNA profiles. DNA profiles are encrypted sets of numbers that reflect a person's DNA makeup, which can also be used as a person's identifier.

CODIS Searches - Who does this DNA profile belong to? Profiles obtained from crime scene evidence are searched against a database of convicted offenders and DNA obtained from other crime scenes throughout the nation.

Blood Stain Pattern Interpretation - What led to the pattern created? Uses scientific principles of biology, chemistry, mathematics and physics to ascertain details and sequence of events of a crime

CODIS

CODIS stands for Combined DNA Index System, a computer software program that operates local, state, and national DNA databases. The software was established and funded by the FBI and was developed to enable public forensic laboratories to create searchable databases of authorized DNA profiles.

Laboratories throughout the country share and compare DNA profiles through the national database known as the National DNA Index System, which is managed by the FBI. All comparisons are conducted electronically.

The goals are to link crimes to each other and identify potential suspects by matching DNA profiles from crime scenes with profiles from convicted offenders and arrestees. Thousands of CODIS matches have linked cases and many cases have been solved by matching crime scene evidence to convicted offender profiles. Searches are conducted and matches are returned to the submitting laboratory.

All profiles entered into CODIS remain indefinitely and are searched every time a new entry is added.

CODIS Entry Criteria

All profiles produced in the Mesa PD laboratory are entered into and searched against the Mesa local DNA database. CODIS entries to be searched against the state and national databases can only be made from DNA evidence collected at a crime scene and that can be attributed to the perpetrator of the crime. DNA profiles from victims cannot be searched against the database.

DNA obtained from items that are in the possession of the suspect cannot be searched against the database (even if it is evidence of the crime). All eligible profiles developed by our laboratory will automatically be entered.

A Scientific Examination Report is issued when a "hit" in the CODIS system occurs.

CONFIRMATION KNOWNS NEEDED: In the case of a CODIS "hit," a known DNA sample from the suspect (unless one has already been collected by a member of the Mesa Police Department) needs to be obtained as a confirmation sample.

WHY? Samples collected for entry into the CODIS database are not evidence (no chain of custody exists). CODIS hits are rarely allowed to be discussed in court. Known samples run by our laboratory can be discussed. Known samples are needed by our laboratory for analysts to testify that the individual matches the crime scene evidence and to offer statistics of the match.

Blood Stain Pattern Interpretation

All members of the Biology Unit are trained and proficiency tested in Blood Stain Pattern Interpretation and are available for crime scene call out. If blood flows during a violent crime, distinctive patterns may result that can provide investigative information about the activities which occurred during the commission of the crime. Theses distinctive patterns occur because of the physical properties of blood and how it reacts when acted upon by physical forces. Blood stain pattern interpretation can be used to determine what caused the patterns left behind, to support or refute a claim of events, and to help describe the events that occurred.

Information that can be obtained:

  • Direction of travel of blood.
  • Level of force used to put the blood into flight.
  • Location of the blood source which was acted upon to create the pattern.
  • Movements during bloodshed.
  • ovements after bloodshed.

Information and Reports from the Biology Unit

SEROLOGY
All requests for DNA testing are first processed for serology. A report will be issued listing all items tested, any body fluids detected, and which items were retained in the laboratory for DNA testing.

DNA
Known reference samples and crime scene evidence are processed at different times. Separate reports are issued for known and unknown samples. The reports reference each other if comparisons are made between them. Multiple analysts may be involved in processing a single case.

WHAT DOES THE REPORT MEAN?
All items analyzed are listed at the top of the report. Each item is divided into a results and conclusions section.

Results Section: A table of the DNA profile is in this section. On the top of the table is the loci (pronounced low-sigh). Underneath is the DNA profile obtained.

Loci are named like this: D8S1179

  • D=DNA
  • 8= chromosome 8
  • S = single copy on chromosome
  • 1179= number that this location was identified on this chromosome.

It's kind of like finding your seat in a stadium where your ticket lists the section, row, and seat number.

Conclusions
The interpretations are listed in the conclusions section. The report will state:

  • If the DNA is from male and/or female origin.
  • If a mixture of DNA is present and the minimum number of contributors.
  • If individual matches = exactly same profile.
  • If individual cannot be eliminated = profile is present in the mixture.
  • No conclusions = individual's profile is partially present and/or missing at some locations of DNA.
  • Does not match = individual's DNA is not the same.
  • Eliminated = individual's profile is not present in the mixture.
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