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Forensic Serology: Blood
The Forensic Serologist
The forensic serologist studies
body fluids such as semen,
saliva, and blood mainly for
identification purposes.
Role(s):
•Establishing type and
characteristics of blood
•Blood testing
•Examination of bloodstains
•DNA typing
•Preparation of court testimony
& evidence
Blood Evidence
Blood is the most well-known and
significant evidence in the modern
criminal justice system.
Blood evidence is important to the
forensic investigator because:
•It can link a victim to a suspect
(Locard’s Exchange Principle]
•Bloodstain patterns can reveal a
great deal about position and
movement during the crime
•It has managed to destroy selfdefense arguments of suspects
Forensic Value of Blood
In forensic science, blood
has always been
considered class evidence.
However, individualized
blood evidence is possible
in the near future. In fact,
in some cases, forensic
serologists were able to
link a single perpetrator
to a bloodstain with
strong probability
estimates.
Individualization of Blood
The potential for the
individualization of blood is
based on the typing of proteins
and enzymes. Blood proteins
and enzymes have the quality
of having polymorphisms or
iso-enzymes, which means they
exist in several forms and
variants.
Most people are familiar with at
least one common
polymorphism in blood: Hb,
which causes sickle-cell
anemia.
Bloodstain Analysis:
The General Questions
During bloodstain analysis, the
forensic investigator uses
these five specific questions
as guidelines for determining
the nature of a crime.
1. Is the sample blood?
2. Is the sample animal blood?
3. If the sample is animal blood,
from what species did it come
from?
4. If the sample is human blood,
what type is it?
5. Can the sex, age, and race of
the source of blood be
determined?
Bloodstain Analysis:
Dried Human Blood?
If severely dried stains are uncovered,
indirect typing should be completed
using techniques such as the absorptionelution test.
During this test, the forensic
scientist adds compatible antiserum
antibodies to a sample, heats the sample
to break the antibody-antigen bonds,
and finally inserts known red blood cells
from standard blood groups to see what
coagulates.
Bloodstain Analysis:
Age, Sex, & Race
-clotting and crystallization tests
can help approximate age
-testosterone and chromosome
testing can help determine sex
-certain racial genetic markers
involving protein and enzyme
tests can help establish race
Blood & Crime Scene:
Wet vs. Dry Blood
Wet blood is more significant than
dried blood because the forensic
scientist can perform more tests
in order to gain insight to the
happenings of the crime. For
example, alcohol and drug
content can be determined from
wet blood only.
Blood begins to dry after three to
five minutes of exposure to air. As
it dries, it changes color from a
deep red towards brown and
black.
Blood & Crime Scene:
Categories of Blood Patterns
A blood smear on the wall or floor can indicate the direction
of force of the blow. The direction of force is always in the
direction towards the tail, or smaller end, of the smear. In
other words, the largest area of the smear is the point of
origin.
Projection of Blood
Forensic investigators can determine
how blood was projected from the
body by examining factors such as:
• Type of injuries
• The order in which the wounds
were received
• Whose blood is present
• The type of weapon that caused the
injuries
• Whether the victim was in motion
or lying still when the injury was
inflicted
• Whether the victim was moved after
the injury was inflicted
• How far the blood drops fell before
hitting the surface where they were
found.
Blood & Crime Scene:
Categories of Blood Patterns
Pools of blood have evidentiary value in collecting a wet
sample.
Drops of blood can reveal the height and angle from
which the blood fell onto the surface.
According to forensic scientists, the blood spatter analysis
claims that blood which falls perpendicular to the floor
from a distance of zero to two feet would create a circular
drop with slightly frayed edges.
Drops from a higher distance would have more distinct
tendrils extending off the edges.
Blood storage
Refrigerated red blood cells have a shelf life of about
forty-two days, and the serum containing
white blood cells can be refrigerated
much longer, almost up to a year.
DNA in blood
DNA can be extracted from
-blood (if white blood cells which always contain a
nucleus are present; red blood cells do not contain
nucleus).
-sperm
-bone marrow
-tooth pulp
-hair roots
Blood & DNA Testing
Blood is used in DNA testing, as shown
by the following steps:
1. Blood samples are collected from the
victim, defendant, and crime scene.
2. White blood cells are separated from
red blood cells.
3. DNA is extracted from the nuclei of
white blood cells.
4. A restrictive enzyme is used to cut
fragments of the DNA strand.
5. DNA fragments are put into a bed of
gel with electrodes at either end.
6. Electric current sorts DNA fragments
by length.
7. An absorbent blotter soaks up the
imprint; it is radioactively treated, and
an X-ray photograph, called an
autoradiograph, is produced.
Blood & Crime Scene
Regardless of what type of analysis is
used on the blood at the crime scene,
care must be taken to handle it
properly and to prevent putrefaction.
Photographs and notes should be taken
before any blood is lifted. Samples
should not be exposed to heat,
moisture, or bacterial contamination
because these factors can shorten the
survival time of proteins, enzymes, and
antigens.
Delays in bringing samples to the lab
must be avoided at all cost, because it
can diminish evidential value.
Court Significance
Experts in bloodstain examination are usually
law enforcement personnel. In certain
jurisdictions, a police investigator or blood
specialist may testify on the core issue
because blood evidence is usually a vital
aspect of the crime scene.
An expert in bloodstain examination has:
•Completed specialized training
•Conducted a sufficient number of
examinations
•Accumulated enough reference patterns
to reinforce an argument