Explore genetic diversity with a cutting edge forensic science experiment! Your students become crime scene investigators as they analyze biological evidence using DNA fingerprinting, a technique that identifies people via genetic differences called polymorphisms. Gel electrophoresis is used to create unique DNA fingerprints from crime scene and suspect samples. A match between samples provides evidence that a certain suspect committed the crime.
WHAT IS A POLYMORPHISM?
In humans, DNA is packaged into 23 pairs of chromosomes. Although most of this DNA is identical between individuals, small sequence differences, or “polymorphisms”, occur at specific locations throughout the genome. The simplest type is the Single Nucleotide Polymorphism (or SNP), a change in a single base pair. Sometimes, such changes affect the number and location of restriction enzyme sites (Fig. 1), resulting in Restriction Fragment Length Polymorphisms (or RFLPs). STRs (Short Tandem Repeats) and VNTRs (Variable Number of Tandem Repeats) are short stretches of repeating DNA sequences. The number of repeats can vary between individuals.
WHAT IS A DNA FINGERPRINT?
If we analyze several different polymorphisms within a person’s genome, we can generate a unique “DNA fingerprint”. After DNA is extracted from biological samples, scientists use the polymerase chain reaction (PCR) to amplify specific places (loci) throughout the genome. The PCR products are analyzed using agarose gel electrophoresis. The PCR products appear on the gel as a series of bands with various sizes (Fig. 1B). Because DNA samples from different individuals produce different patterns of bands, scientists can use a DNA fingerprint to distinguish between individuals. Since polymorphisms are inherited, DNA fingerprints can be used to determine paternity/maternity (and other familial relationships).
HOW DO FORENSIC SCIENTISTS USE DNA FINGERPRINTS?
The best-known application of DNA fingerprinting is in forensic science. DNA fingerprinting techniques are utilized to analyze blood, tissue, or fluid evidence collected at accidents and crime scenes. The DNA fingerprint from a crime scene can be compared with the DNA fingerprints of different suspects or those stored in CODIS (COmbined DNA Index System), a computer database of DNA fingerprints collected from convicted offenders, arrested persons, and crime scene evidence and missing persons.
A match between the crime scene DNA and a suspect’s DNA at a single locus does not prove guilt, nor does it rule out innocence. Therefore, multiple loci are tested. For example, the DNA fingerprints stored in CODIS contain data on thirteen loci. The odds of a match at all thirteen loci are less than one in a trillion.