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Crime Scene Analysis with Blood Typing
Introduction
Although DNA is used to link a suspect to a crime scene, blood type can also help determine whether or not to
exclude a suspect. Blood type is determined by three alleles (IA, IB, and i). IA and IB are codominant to each other but
are both dominant to i. IA is the “type A” allele, IB is the “type B” allele, and i is the “type O” allele. In order to be type
AB, one needs both the ) IA and IB alleles.
In this lab, you will determine the blood types of an unknown crime scene sample of blood and the blood of four
suspects. Blood antigens invoke an immune response in foreign blood. Antibodies that react with antigens cause
agglutination of the blood. Agglutination of blood type A (A antigens) would occur in the presence of anti-A
antibodies. Agglutination of blood type B (B antigens) would occur in the presence of anti-B antibodies. Think about
the antigens present in AB blood and O blood and determine in the pre-lab questions which antibodies would cause
these types to agglutinate.
Rh factor is another antigen type other than A & B. If Rh antigen is present in blood, anti-Rh antibodies will cause
agglutination (anti-D serum represents anti-Rh antibodies).
When typing blood, anti-serums are used. These are the antibodies made by the immune system to attack foreign
blood types. If the blood agglutinates, or clumps up, when the anti-serum is added, it indicates that that blood
type is present. Positive tests may also appear to be opaque or cloudy. If the blood remains clear after stirring in the
anti-serum, it is negative for that blood type. A blood typing tray has 3 wells: A, B, and Rh. To test someone’s blood,
a drop of his or her blood is placed in all 3 wells, and the appropriate anti-serum is added. From the blood’s reaction
(clumping or not), you can determine the blood type.
Pre-lab
1. What is meant by the term “agglutination?”
2. If the blood remains clear after stirring in the anti-serum, is it a positive or negative test?
3. What indicates a positive test?
4. Which antibody or antibodies would cause agglutination with type AB?
5. Which antibody or antibodies would cause agglutination with type O?
6. Give the blood type for the following tests:
Well
A
B
Rh
Agglutination
(√, means
positive)
√
√
Blood
type?
Well
A
B
Rh
Agglutination
(√, means
positive)
√
Blood
type?
Well
A
B
Rh
Agglutination
(√, means
positive)
Well
Blood
type?
Agglutination
(√, means
positive)
√
A
B
Rh
√
√
Blood
type?
LAB ACTIVITY
Materials: Simulated blood, simulated blood anti-serum, simulated anti-Rh serum, toothpicks, blood typing plates
Procedure: Analyze the following blood types from 4 possible suspects for the crime.
1. Obtain blood typing trays and make sure they are clean—if not, wash and dry them.
2. Using one blood typing tray, place a drop of “Crime Scene” blood in the A and B wells of the plate.
3. Next, add a drop or two of anti-A serum (blue) to the A well and a drop or two anti-B serum (yellow) in the B
well and a drop or two of anti-Rh serum (clear) to the Rh well.
4. Use a clean toothpick for each, stir each well for 30 seconds.
5. Record the results in Data Table 1.
6. Repeat steps 1-5 for each of the potential suspects: Mr. Castle, Mr. Knight, Mr. Gold and Mr. Blue. Be sure to
use clean blood typing trays and toothpicks for each!
7. Wash and dry your blood typing trays and throw all toothpicks in trash can.
Data Table 1
Person
Agglutination
A
B
Rh
Blood Type
Crime Scene
Mr. Castle
Mr. Knight
Mr. Gold
Mr. Blue
When testifying in court, a forensic scientist would need to be able to give an explanation in “layman’s terms”’
to the jury that is easy to understand. Eliminate each of the potential suspects and give an explanation as to
why they can or cannot be the individual whose blood was left at the crime scene. Circle the suspect(s) that has
(have) a blood type consistent with the blood at the scene.
Father
Mr.
Castle
Mr.
Knight
Explanation
Mr.
Gold
Mr. Blue
Post-Lab Questions:
1. At 1:00 a.m., someone breaks a window in the back of a store and robs the safe. On the way out, the
thief cuts himself on a piece of broken glass. You are a forensic detective called to the scene. You test a
sample of blood left behind by the thief. It is O-. While you are there, police bring in a suspect with a
cut forearm who was arrested just three blocks from the store. You take a sample of the suspect’s
blood and mix it with anti-A. You immediately know that the suspect is not the person who cut himself
on the broken glass in the store. Explain how you came up with this conclusion.
2. Suppose the suspect’s blood does not agglutinate when tested with anti-A or anti-B, but does
agglutinate when tested with anti-Rh. Determine whether or not this would connect the suspect with
the crime scene? Explain your answer.
3. Tom and Jane participate in a Red Cross blood drive. Both are first-time donors. As part of the
screening process, their blood is typed. Tom is A+ and Jane is AB+. What blood group antigens are
found in Tom’s blood? Jane’s blood?
4. Tom and Jane’s blood donations are sent to a processing center where their blood cells are separated
from their plasmas. Both their separated cells and plasmas are then sent to a hospital. A blood
researcher whishes to use Tom’s blood in an attempt to extract and identify the A antigen. Determine
whether she should attempt the extraction process on his blood cells or on his plasma and explain your
answer?