01 - Forensic Escape Room: Design Your Own Biotech Adventure
BLOOD TYPING Blood typing is an immensely important clinical procedure. It is one of the first procedures performed during blood transfusions and surgery, and is also important in forensic science. Blood typing is an example of an agglutination assay, the precipitation of antigens on red blood cells and antibodies in the blood. When both components are present at a similar concentration they are in a state known as
equivalence. In an equivalent state, neither the antibody nor the antigen is in excess, and the antigen-antibody complex forms large networks that precipitate out of solution. Impor- tantly, the precipitate is easy to detect by eye, making agglutination assays both easy and cost-effective to perform. The most common blood typing system relates to the presence of the A and B antigens on the red blood cells. This system, known as the ABO blood types, produces four
Antigen on Red Blood Cells
Antibody in Blood
Percentage of Population
Blood Type
A B AB O
A B A & B O
anti-B anti-A none anti-A & anti-B
42% 10% 4% 44%
Figure 1: Types of Blood in the Population
possible blood types: A, B, AB, and O (Figure 4). Individuals with only A antigens will have type A blood, while someone with B antigens has type B blood. The A and B antigens are co-dominant, so a person can have both antigens on their red blood cells, leading to the AB blood type. If an individual has neither A nor B antigens they have type O blood. A person with type A blood will recognize red blood cells with the A antigen as “self”. However, if that person gets a blood transfusion with type B blood, the new red blood cells will be recognized by the immune system as foreign and will cause an immune response. Antibodies targeting the B antigens (anti-B antibodies) will bind to the B antigen on the transfused cells and agglutinate. In many cases, this severe immune response can be deadly. Therefore, it is important for hospitals and clinics to maintain records of patient blood types. The same reaction that can lead to severe immune responses in a patient is used for clinical and foren- sic blood typing experiments. For example, type B blood can be easily recognized by the agglutination between the anti-B antibodies and the B antigen. When something that is suspected to be blood is found at a crime scene, detectives will work quickly to secure the evidence and send it to a forensic lab for testing. In the lab, forensic scientists will perform presumptive and confirmatory tests for blood, potentially recommending additional testing such as DNA profiling. DNA FINGERPRINTING In humans, DNA is packaged into 23 pairs of chromosomes that are inherited from an individual’s bio- logical parents. Although most of this genetic material is identical in every person, small differences, or “polymorphisms”, in the DNA sequence occur throughout the genome. For example, the simplest differ - ence is a Single Nucleotide Polymorphism (or SNP). Changes in the number and location of restriction enzyme sites result in Restriction Fragment Length Polymorphisms (or RFLPs). Short repetitive stretches of DNA at specific locations in the genome can vary in number to produce STRs (Short Tandem Repeats) and VNTRs (Variable Number of Tandem Repeats). Although most polymorphisms occur in non-coding regions of DNA, those that disrupt a gene can result in disease. Medical diagnostic tests can identify specific polymorphisms associated with disease. Analyzing several different polymorphisms within a person’s genome generates a unique DNA “finger - print”. DNA fingerprints can allow us to distinguish one individual from another. Because polymorphisms are inherited, DNA fingerprints can also be used to determine paternity/maternity (and other familial relationships). The best-known application of DNA fingerprinting is in forensic science. DNA fingerprint - ing techniques are utilized to interpret blood, tissue, or fluid evidence collected at accidents and crime scenes. After DNA is extracted from these samples, forensic scientists can develop a DNA fingerprint. The DNA fingerprint from a crime scene can then be compared to the DNA fingerprints of different suspects. A match provides strong evidence that the suspect was present at the crime scene.
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