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Name_____________________________________ Block__________________ Date_____________________ Forensics Mystery Blood Typing Lab Scenario: Police are called to a house where they discover a body near the back door. There is a fresh pool of blood by the victim’s head. Officers question the neighbors and learn that a plumber and an electrician made service calls to the house just hours before. They were able to trace both. They recover a shirt with a dark stains from the service van of the plumber and rags with similar dark stains from electrician’s van.
Your forensics lab receives the following items to test: Sample 1: Stained cloth from plumber’s van Sample 2: Stained cloth from the electrician’s van Sample 3: Blood collected from under the victim’s head Test Background: The phenolphthalein test is a highly sensitive, being capable of detecting blood spots practically invisible to the eye. In an actual criminal case, a positive phenolphthalein test would be followed by tests to determine the species that produced the blood and if these tests indicate the blood is human, by tests to determine ABO, Rh and other blood groups.
There is no point in pursuing typing and species identification procedures if the phenolphthalein test is negative. The phenolphthalein test is not absolutely specific for blood because most plants and some animal cells can also cause a positive reaction. Discussion Questions: 1. Does a positive phenolphthalein test prove that a stain is caused by blood? 2. Does a positive phenolphthalein test prove that a stain is caused by human blood? Procedure 1: 1. Test the shirt and rags to determine if the stains are caused by blood. 2. Place a drop of phenolphthalein/peroxide solution on each stain. 3.
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Observe if a color change occurs. A purple-violet color reaction indicates that blood (heme) is present (positive reaction). Absence of this reaction indicates that blood is not present. 4. Record your results: Color reaction for Sample 1:_________________________ Color reaction for Sample 2:_________________________ Based on the results of this test, which person, the plumber or the electrician becomes your prime suspect? ____________________________________________________________________________ Additional Information: When confronted, your chief suspect claims to have a nosebleed, thus accounting for the blood stain.
Investigators have now been able to recover a blood sample (Sample 4 ) from the suspect’s van. It is suitable for blood typing. You ask and receive a blood sample from the suspect (Sample 5). Proceed to Procedure 2. Test Background: Blood groups are created by molecules present on the surface of red blood cells. The table shows the four ABO phenotypes (“blood groups”) present in the human population. When red blood cells carrying one or both antigens are exposed to the corresponding antibodies, they agglutinate; that is, clump together.
People usually have antibodies against those red cell antigens that they lack. The ABO Blood Groups Blood Group| Red Cell Antigen (Protein on blood cell)| Serum Antibody (Produced by Body)| A| A| Anti-B| B| B| Anti-A| AB| A and B| Neither| O| Neither| Anti -A and Anti -B| Discussion Questions: 1. How do you determine a person’s blood type? 2. Can a person’s blood type prove or disprove that he/she committed a crime? How? Procedure 2: 1. Add a drop of synthetic anti-A (blue) to the well labeled A. Replace cap. . Always replace the cap on the vial before opening the next vial to prevent cross contamination. . Add a drop of synthetic anti-B (yellow) to the well labeled B. Replace cap. 3. Add a drop of synthetic anti-Rh serum (clear) to the well labeled Rh. Replace cap. 4. Using the dropper vial, place a drop of Sample 3 (the victim’s blood) in each well of the blood typing slide. DO NOT TOUCH THE WELL! 5. Replace the cap on the dropper vial. 6. Using a different color mixing stick for each well (blue for anti-A, yellow for anti-B & white for anti –Rh), gently stir the synthetic blood and anti-serum drops for 30 seconds.
Remember to discard each mixing stick after a single use to avoid cross contamination. 7. WAIT 60 Seconds. 8. Carefully examine the thin films of liquid mixture left behind. If the film remains uniform in appearance, there is no agglultination (no clumps, negative reaction). If the film as clumps, agglutination has occurred (positive reaction) 9. Determine blood type of the sample using the data table below. Answer yes or no as to whether agglutination occurred in each sample. A positive agglutination reaction indicates blood type. 10. Record the results for each blood sample in the data table.
Data Table | Sample 3| Sample 4| Sample 5| Anti-A| | | | Anti-B| | | | Rh| | | | Blood type| | | | 11. Then repeat steps 1-9 for Samples 4 and 5. Record each results on table. 12. Thoroughly rinse the blood typing slides and return all materials. Conclusion: Based on your results, did the blood collected from the suspect’s van come from the suspect? ____________________________________________________ Could this blood have come from the victim? ____________________________ What can you conclude from your results? (Remember to cite your data). __________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ______________________________________________________________________________________________________________________________________________________________________
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