By: Monica Ye & Lilliana Mendoza
Mentor: Dr. John Molina
Bronx Community College
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Blood is a mixture of cells, enzymes, proteins, and inorganic
substances.
The fluid portion of blood is called plasma, which is mostly
contains nutrients and dissolved proteins.
The other portion of blood is the cellular phase. This contains
red blood cells, white blood cells, and platelets.
Antigens are proteins located on the surface of red blood
cells and are responsible for blood type characteristics.
The most important types of antigens are A-B-O.
Type A has A antigens in one’s red blood cells. Type B has B
antigens. Type AB has AB antigens, and type O has neither A
nor B antigens.
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For every antigen there is a specific antibody that
will react with the antigen to form clumps known as
agglutination. For example, if anti-B is added to
red blood cells carrying B antigen; there will be a
reaction.
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The blood that is found a crime scene contain
Deoxyribonucleic Acid (DNA).
DNA is found in the nucleus of cells which contains
the genetic material of living organisms.
DNA is used to identify the victim(s) and the killer
involved.
Blood can also be used to determine the sequence of
how a crime occurred.
The blood that is found can be used to determine
what weapon (if any) was used in the crime.
Blood can also help identify the origin of the blood.
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When blood is found at a crime scene, it is possible that the
substance may not be blood at all.
A preliminary or screening test procedure is used to determine if
the substance is blood; this preliminary or screening test
procedure is called Presumptive Testing.
Presumptive Testing is based on the reactivity of hemoglobin, a
protein found in red blood cells used to carry oxygen throughout
body. Two different chemicals are used in this test:
Phenolphthalein and O-toluidine.
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How are the chemicals used?
- A sample of the blood is taken on a Q-tip and a small
amount of the chemical is dropped on to the sample.
- If the substance is blood, when Phenolphthalein is used;
the sample will turn pink. If O-toluidine is used; a green-blue
color will appear. When these colors appear, it doesn’t
necessarily mean that it is blood. The blood sample needs to
undergo another procedure that confirm the presence of
blood.
Both Phenolphthalein and O-toluidine reacts to the
Hemoglobin, which is an oxygen- carrying protein in the red
blood cells.
A sample of the blood is placed on a slide
and examined under a microscope.
  If the substance is blood crystal-like shapes
will form.
  If the substance is not blood no crystals will
form.
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After we confirm that the substance found is blood at the
crime scene; PCR analysis is used. PCR stands for
‘polymerase chain reaction’. This is a technique that is
used to create millions of copies of DNA just using a
single sample of DNA.
This technique is used by forensic scientists to perform
DNA analysis on really small samples; such as skin cells.
So from the blood substance, DNA is extracted and then
analyzed using this procedure.
Other tests may include: STR (Short Tandem Repeat
testing), RFLP (Restriction Fragment Length
Polymorphism).
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Blood spatter is a group of blood stains resulting from one or more
injuries.
Spatters are produced in different ways; such as stabbing, gunshots,
beatings, cast-off blood, and splashing.
Careful study of these spatters can help forensic scientists to figure:
-out the origin of the blood stains
-type of weapons used
-the direction which the object struck the victim
-movements of the victim and the suspect
-the number of blows and gunshots the victim received
-sequence of events
There are three basic types of blood stains:
- Passive: are blood drops created or formed by the force
of gravity alone.
- Transfer: is created when a wet, bloody surface comes
into contact with another surface.
- Projected: are created by more force or action greater
than gravity.
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-When a blood drop is dropped from a 90 degree angle,
the drop is round and the length as well as the width
would be equal.
-When blood falls at an angle less than 90 degrees, its
shape would resemble a teardrop.
-The pointed end of the blood stain is called the “tail”
and it would always point in the direction of the travel
of the blood.
Example: SIN  = Width (a) 1.5cm
Length (c) 3.0cm
Width (a) 1.5cm = 0.5 = SIN 
Length (c) 3.0cm
  = 30O
-At a 90 degree angle, the
blood drop is circular and
there won’t be any short or
long axes.
-Below 75 degrees, the spines
become more prominent on
the side of the spatter
opposite the angle of impact.
-At highly acute angles; usually
below 40 degrees, the blood
drop begins to form tails and
a second spatter coming
from the first one.
-The angle of impact can be
determined by taking the
inverse sine of the width over
the length of the blood stain.
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Slemko, J. "BLOODSTAIN PATTERN ANALYSIS TUTORIAL." Homepage. Web. 28
July 2009. <http://www.bloodspatter.com/BPATutorial.htm>.
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Forensic Science Service Ltd. "Forensic Science Service - Blood Pattern Analysis." Forensic Science
Service - Home Page. Web. 28 July 2009. <http://www.forensic.gov.uk/html/services/
analytical-solutions/blood/>.
Mindrelief.net. "Blood in the Crime Scene." Custom essay, term paper, research paper writing service.
Web. 30 July 2009.
<http://www.mindrelief.net/blood.html>.
Old Spring Harbor Laboratory. "Cutting, Pasting, & Copying DNA & the Recombinant DNA Controversy."
DNA Interactive: Discovering the DNA Structure and beyond. Web. 03 Aug. 2009.
"RFLP Definition." Www.FoodSafety.gov - Gateway to Government Food Safety Information. Web. 05
Aug. 2009. <http://www.foodsafety.gov/~frf/rflp.html>.
"RFLP Definition." Www.FoodSafety.gov - Gateway to Government Food Safety Information. Web. 05
Aug. 2009. <http://www.foodsafety.gov/~frf/rflp.html>.
PBS. "A Science Odyssey: DNA Workshop: Replication." PBS. Web. 03 Aug. 2009.
www.pbs.org/wgbh/aso/tryit/dna/replication.html>.
<http://
"RFLP Definition." Www.FoodSafety.gov - Gateway to Government Food Safety Information. Web. 05
Aug. 2009. <http://www.foodsafety.gov/~frf/rflp.html>.
Think Quest. "Forensic Science | Blood Analysis." Oracle ThinkQuest Library. Web. 05 Aug. 2009.
<http://library.thinkquest.org/04oct/00206/lo_pti_blood_analysis.htm>.
http://www.bloodspatter.com/terminology.pdf
http://www.nfstc.org/links/animations/images/blood%20spatters.swf
Dr. Thomas Brennan
o  Dr. John Molina
o  Dr. Sat Bhattacharya
o  Bronx Community College
o  National Science Foundation
o  Harlem Children Society
o  Harlem Children Society Staff
o  Audience
o