Using Blood Tests to Identify Criminals

Copyright, 2012, by Drs. Jennifer Doherty and Ingrid Waldron, Department of Biology, University of Pennsylvania

Who Killed Shamari Davis?

Background

Shamari Davis was a 20-year-old college freshman who was majoring in Physical Therapy. She paid for school by working as a personal trainer at a local gym. Shamari had been promoted to head personal trainer at the gym just before she was killed.

Crime Scene

The body was found in the women’s locker room of the gym at 1:00 a.m. by the night janitor, Harvey Willis. The victim had been strangled and was wearing a robe. There were signs of a struggle in the room and the glass door of the shower was broken and had traces of blood on it. The victim was pronounced dead at the scene and the coroner suggested that the time of death was at least three hours before the body was found.

Criminal Investigation

Shamari’s co-worker Daleesha Jones told police that Shamari was a newer employee who did not deserve her recent promotion and only got it because she spent a lot of time with their boss, Steve O’Hare. When asked if he knew if Shamari had problems at work, Steve told Police that Shamari had complained to him that one of her fitness clients, Mike Reed, kept asking her out and wouldn’t take no for an answer.

Blood Analysis

Obviously a real crime investigation would use many clues, but your investigation will be based on the simplest type of blood testing, namely testing for blood types A, B, O, and AB, for the blood sample found at the scene and for each of the possible suspects.

★ No individual can change blood types, and blood type does not change with age. Explain why.

In order to test blood type, you mix a sample of the blood with two different types of antiserum — one which contains anti-A antibodies and one which contains anti-B antibodies. The reactions between the antibodies in the antiserum and the corresponding antigens on the red blood cells in the blood sample result in clumping.

★ Which types of blood have the antigens that will react with anti-A antibodies?

★ Which types of blood have the antigens that will react with anti-B antibodies?

 Before you carry out the blood type tests, fill in the following chart that will help you to identify the blood type of each individual.

Procedure

1. Place your dish with the test wells on a piece of white paper, and put two drops of the blood of one suspect on both the A and B wells of the dish.
2. Place two drops of anti-A antibody solution on the drop of blood in the A well and place two drops of anti-B antibody solution on the drop of blood in the B well.
3. Mix the blood sample with the added anti-A antibody solution with one end of the toothpick. Mix the blood sample with the added anti-B antibody solution with the other end of the toothpick. Discard each toothpick after you use it.
4. Record both reactions in the table below, and record the blood type of the individual.
5. Repeat the procedure, steps 1 through 4, for each blood sample.
6. Compare the blood types for the samples from the victim and each suspect to the blood type from the broken shower door glass at the scene of the crime. Use your observations to suggest who committed the murder.

Investigator’s Report

★ Describe the circumstances which you believe led up to the crime, the time of the crime, and the individual that you believe is guilty of the murder. What evidence supports your conclusions?