## DNA profiling

DNA analysis for human profiling (also called DNA fingerprinting) was first published by Alec Jeffreys from the University of Leicester (Great Britain) in 1985. DNA fingerprinting played an important role in solving the criminal case of the murders of two young women in the English county of Leicestershire. During the investigation, Richard Buckland, who was only 17 years old at the time, was accused based on the matching of the blood group and the relatively rare property of excreting blood antigens into body fluids. Under the pressure of the investigation, he finally confessed to one of the crimes. After some time, however, the investigators learned about the possibility of DNA analysis through the research of Alec Jeffreys. After comparing the RFLP profile from the crime scene samples to that of Richard Buckland, it was found that they did not match. So, the police reopened the case and summoned approximately 5,000 men from the surrounding cities to take samples. Those whose samples matched the crime scene samples by blood type were subjected to RFLP analysis. However, to the great disappointment of the investigators, they did not find a match. After some time, a woman came to the police claiming that she had heard Colin Pitchfork, a baker at a local inn, persuade another man to go and hand over the sample to the police instead of him. Based on this testimony, the police arrested Colin Pitchfork, conducted a DNA analysis, and found that his RFLP profile matched the DNA from the crime scene samples. It was a great success because, for the first time in history, DNA analysis contributed to the clarification of such a serious crime and the conviction of the real murderer.

An even more accurate (and thus more informative) method of DNA analysis in the identification of people are DNA length polymorphisms (Figure 15.4). In STR (short tandem repeats) length polymorphisms, two individuals differ in the number of repeats of short DNA segments. The advantage of this approach is that STR polymorphisms are much more diverse than SNP polymorphisms, so they allow many different people to be distinguished from each other. Thus, there are two possible combinations from the RFLP analysis (either the DNA is cleaved or not), while with STR polymorphisms there are many more possible combinations, depending on the variability of the polymorphism in the population. For example, a certain individual may have five repetitions of one STR polymorphism on one chromosome and seven on the other, and another individual may have eight repetitions of the given polymorphism on one chromosome and four repetitions on the other chromosome (Figure 15.4). STR polymorphisms are usually analysed by amplifying a given section using PCR (similar to RFLP), but their detection is most often done by capillary electrophoresis. We also refer to this technique as fragmentation analysis and it is much more accurate when distinguishing DNA fragments with small differences in length.

Currently, 13 (in some cases up to 20) different STR polymorphisms are investigated in standardised forensic laboratories for human identification, while the probability of a match between two unrelated people in such an analysis is up to 1:1012, that is, close to zero. In 1998, the international CODIS database was established by the American FBI agency, in which the DNA profiles of all individuals investigated in any court proceedings are collected. The CODIS software is also used by a large number of countries across the world for their own law enforcement, but also to identify missing and unidentified individuals.