Dr. Bill Bass and Dr. Neal Haskell are forensic anthropologists who use science to help law enforcement solve crimes. Dr. Bass studies how bodies decay over time. Dr. Neal Haskell collects the insects he finds at the crime scene to determine a victim’s time of death.
In this activity, students will learn about another form of forensics: blood typing. Students conduct experiments with simulated blood to determine the blood type of each sample. Using the results of the experiment, students will determine if a blood sample found at the scene of a crime matches the blood sample of a possible suspect.
Grade Level: 6th – 8th grade
Subject Matter: Life Science
National Standards: NS.5-8.1, NS.5-8.3
SciFri Video: The Body Farm
In 1992, Science Friday investigated how forensic anthropologists solve crimes using bugs. Now you and your students can take a tour of “The Body Farm,” where scientists study body decomposition. Beware: the work is as gruesome as it sounds.
Red food coloring
Green food coloring
Clear plastic cups
16 plastic droppers, four for each workstation for each experiment
Small paper cups
Clean sheets of paper, enough for each student
Note: In order to reduce the amount of preparation time and materials needed, set up this lesson as a collaborative learning experience with four workstations – one for each blood type to be tested. At the end of the first experiment, collect results from each of the four workstations and discuss them with students. For the second experiment, each workstation will be able to conduct the same experiment, using the Suspect’s blood sample and the Crime Scene blood sample. If you have a smaller class, you can modify the number of samples to suit class size.
Prep simulated blood:
1. Label six plastic cups according to one of the blood types in the chart below: A, B, AB, O, Suspect, and Crime Scene.
2. Make five cups of simulated blood by combining red food coloring with milk until it is bright red, and then adding a couple of drops of green food coloring for a dark red color. Split the simulated blood evenly into each of the blood type cups labeled A, B, AB, O, Suspect, and Crime Scene.
3. Fill five paper cups with vinegar, and six more with water.
Prep simulated antibody samples for Experiment 1:
1. Make the eight antibody samples for blood types A, B, AB, and O by labeling eight plastic cups according to the chart columns for antibody Anti-A and antibody Anti-B. You will need four of each sample, enough for each workstation.
2. Add ½ cup of the appropriate indicator to each cup.
Prep simulated antibody samples for Experiment 2:
1. For the Suspect and Crime Scene blood samples, make four of each antibody (one for each workstation), corresponding to the chart columns antibody Anti-A and antibody Anti-B.
1. Arrange the tables and chairs in the classroom to function as four workstations. Students can work in pairs or as a group.
2. Use the workstation column in the chart below to place the appropriate samples at each workstation.
3. For Experiment 1, each workstation should be assigned a blood type and include:
- A sample of the assigned blood type (A, B, AB, or O).
- The Anti-A sample created for that blood type.
- The Anti-B sample created for that blood type.
- Empty paper cups, toothpicks.
4. For Experiment 2, each workstation should include:
- Suspect blood type sample and the corresponding Anti-A and Anti-B sample.
- Crime Scene blood sample and the corresponding Anti-A and Anti-B sample.
- Empty paper cups, toothpicks.
5. Make sure that there is one plastic dropper for each blood sample and one for each antibody, to prevent cross contamination.
6. For Experiment 2, make a blackboard or wall chart that looks like this:
Antigen: large molecule on the surface of red blood cells.
Antibody: a protein produced in response to the presence of an antigen.
Agglutination: the clumping of red blood cells which occurs when antibodies react to antigens.
Blood typing: the process of determining a person’s blood type, whether A, B, AB or O.
Forensics: a combination of sciences used by law enforcement to solve crimes.
What To Do
1. Begin the lesson by having the students watch the Science Friday video, “The Body Farm.” Begin a discussion with students on how Dr. Bill Bass and Dr. Neal Haskell are able to solve criminal cases by looking at body decomposition and insects found at the crime scene. What other types of forensic methods can students name that can be used to solve a case? Tell students that they are going to help solve a recent crime by learning how to use blood typing to identify a criminal.
2. Introduce the crime scene to the students by describing the following details:
Two nights ago, a burglary was committed at the local high school. The perpetrator entered the school building through the emergency exit on the roof and stole six laptops from the computer lab. The laptops were stored in a locked glass cabinet. The perpetrator broke the glass in order to unlock the cabinet. In so doing, the perpetrator cut his or her arm on shards of broken glass, and left drops of blood behind. Last night, police arrested a man who was caught asking people on the street if they were interested in buying a cheap laptop. Police were able to get a warrant for a sample of the suspect’s blood.
3. Ask students to describe what they know about blood types. Tell students that there are four possible human blood types: A, B, AB, and O. Do they know their own blood type? How could they find out what type they are? How do they think blood typing be used as evidence against a suspect?
4. Tell students that they will conduct two experiments. The first experiment will be used to observe each blood type’s reaction to an antibody. The second experiment will use the results from the first experiment to determine if the suspect’s blood matches a blood sample collected from the crime scene.
Experiment One: Blood Typing
1. Divide students into four groups and assign each group to one of the blood type workstations. Hand out sheets of paper to each student, and inform students that they will be responsible for observing and recording the reactions of the blood type assigned to them. Remind students that they will be testing their assigned blood type twice, once with antibody Anti-A and once with antibody Anti-B.
2. Using the appropriate plastic dropper, have students place three to four drops of simulated blood into a small paper cup. Then have students add five drops of antibody Anti-A into the same paper cup. Mix the antibody and blood with a toothpick.
3. Have students look for clumping or agglutination of the blood. Were there any reactions? Students should record their observations on their sheets of paper.
4. Students should repeat Steps 2-3 with the antibody Anti-B and record the reactions on their paper.
5. As a class, chart the results on the board, using the table below as a template. Add a check mark (“√”) to the chart when there was a reaction, and an “X” when there was no reaction.
6. Ask students how they can use this information in relation to the case. How is a chart that lists the reaction of the antibodies to the blood type helpful?
Experiment Two: Matching Blood Types
1. Have students test the Crime Scene blood sample, firstly with the antibody Anti-A and then with antibody Anti-B. Students should use sheets of paper to record the reaction of the Crime Scene blood sample to the antibodies. Use the classroom chart to determine the blood type of the Crime Scene sample.
2. Have students test the Suspect’s blood sample, firstly with the antibody Anti-A and then with the antibody Anti-B. Record the reaction of the Suspect’s blood sample to the antibodies. Use the classroom chart to determine the blood type of the Suspect blood sample.
3. Using the results gathered in the last two steps, ask students to determine if the Suspect’s blood type matches the blood type found at the scene of the crime.
4. Discuss with students the strengths and weaknesses of the evidence. Does the evidence completely prove or disprove that the Suspect committed the crime?
A person's blood type is defined by the presence or absence of large molecules called antigens on the surface of their red blood cells. Antibodies react with antigens that are foreign to our bodies. These antigens can include bacteria, viruses, pollen or somebody else’s red blood cells. Agglutination, or the clumping of red blood cells, can occur when antibodies react with antigens.
There are four possible human blood types – A, B, AB, and O. Depending on the blood type, each may or may not have two red blood cell antigens, A or B. Type A has antigen A, type B has antigen B, type AB has both antigens and type O has no antigens.
In this investigation, antibodies Anti-A and Anti-B were added to various simulated blood samples to observe whether agglutination would occur. The curdling of milk when mixed with vinegar was used to simulate and represent the clumping of red blood cells. In Experiment 1, students are practicing blood typing protocol to determine how each blood type reacts to an antibody. In Experiment 2, the results collected from each blood type allow for a comparison of the blood collected at the Crime Scene and the Suspect’s blood sample. This data are used to determine if the two samples are the same blood type.
Although DNA evidence is considered to be a more persuasive proof of guilt or innocence, blood typing is a less expensive and less time consuming method that can be used in the initial stages of investigation of a crime. In this case, the Suspect’s blood type (type O) did not match the sample from the Crime Scene (type B).
Topics for Science Class Discussion
- How do antigens and antibodies relate to blood transfusions? Why is it important to know your blood type?
- What could be some possible next steps for a forensic scientist to take if the Suspect’s blood type matched the blood from the Crime Scene?
- What are some other ways scientists help solve crimes?
Extended Activities and Links
Have students search the local newspaper or the Internet for criminal investigations that use forensic evidence. Students should research the methods used in that particular investigation and present their findings to the class.
Have students research forensic entomology, and find out how scientists use insects such as flies, maggots, and beetles to reveal details about a crime scene.
Listen to the SciFri Podcast “Call it CSI: Wilderness” about animal forensic investigators and discuss how they compare to human forensics investigators:
Listen to the SciFri Podcast “The Body Farm” for an interview with Bill Bass.
Play an online game that simulates how blood typing is used for blood transfusions:
Explore the interactions between antibodies and antigens in this lesson plan:
This lesson plan was created by the New York Hall of Science in collaboration with Science Friday as part of Teachers Talking Science, an online resource for teachers, homeschoolers, and parents to produce free materials based on very popular SciFri Videos to help in the classroom or around the kitchen table.
The New York Hall of Science is a science museum located in the New York City borough of Queens. NYSCI is New York City's only hands-on science and technology center, with more than 400 hands-on exhibits explore biology, chemistry, and physics.