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We assume that the person having this DNA profile received the 11 and 13 alleles at random from each parent. Therefore, the probability that this person received allele 11 from the mother and allele 13 from the father is expressed as p X q = pq. In addition, the probability that the person received allele 11 from the father and allele 13 from the mother is also pq. Hence, the total probability that this person would have the 11, 13 genotype at this locus, by chance, is 2pq. As we see from Table ST 5.2, 2pq is 0.102 or approximately 10 percent. It is obvious from this sample that using a DNA profile of only one locus would not be very informative, as about 10 percent of the population would also have the D5S818 11, 13 genotype he discrimination power of the DNA profile increases when we add more loci to the analysis. The next locus of this person’s DNA profile (TPOX) has two identical alleles— the 11 allele. Allele 11 appears at a frequency of 0.243 in this population. The probability of inheriting the 11 allele from each parent is p X p = p”. As we see in the table, the genotype frequency at this locus would be 0.059, which is about 6 percent of the population. If this DNA profile contained only the first two loci, we could calculate how frequently a person chosen at random from this population would have the genotype shown in the table, by multiplying the two genotype probabilities together. This would be 0.102 x 0.059 = 0.006. This analysis would mean that about 6 persons in 1000 (or 1 person in 166) would have this genotype. The method of multiplying all frequencies of genotypes at each locus is known as the product rule. It is the most frequently used method of DNA profile interpretation and is widely accepted in U.S. courts. By multiplying all the genotype probabilities at the five loci, we arrive at the genotype frequency for this DNA profile: 9 x 107’. This means that approximately 9 people in every 10 million (or about 1 person in a million), chosen at random from this population, would share this 5-locus DNA profile. The Uniqueness of DNA Profiles As we increase the number of loci analyzed in a DNA profile, we obtain smaller probabilities of a random match. Theoretically, ifa sufficient number of loci were analyzed, we could be almost certain that the DNA profile was unique. At the present time, law enforcement agencies in North America use a core set of 20 STR loci to generate DNA profiles. Using this 20-loci set, the probability that two people selected at random would have identical genotypes at these loci would be approximately 1 x 10-28. Although this would suggest that most DNA profiles generated by analysis of the 20 core STR loci would be unique on the planet, several situations can alter this interpretation. For example, identical twins share the same DNA, and their DNA profiles will be identical. Identical twins occur at a frequency of about 1 in 250 births. In addition, siblings can share one allele at any DNA locus in about 50 percent of cases and can share both alleles at a locus in about 25 percent of cases. Parents and children also share alleles, but are less likely than siblings to share both alleles at a locus. When DNA profiles come from two people who are closely related, the profile probabilities must be adjusted to take this into account. The allele frequencies and calculations that we describe here are based on assumptions that the population is large and has little relatedness or inbreeding. If a DNA profile is analyzed from a person in a small interrelated group, allele frequency tables and calculations may not apply. DNA Profile Databases Many countries throughout the world maintain national DNA profile databases. The first of these databases was established in the United Kingdom in 1995 and now con- tains more than 6 million profiles. In the United States, both state and federal governments have DNA profile databases. The entire system of databases along with tools to analyze the data is known as the Combined DNA Index System (CODIS) and is maintained by the FBI. As of August 2018, there were more than 17 million DNA profiles stored within the CODIS system. These include the convicted offender data- base, which contains DNA profiles from individuals con- victed of certain crimes, and the forensic database, which contains profiles generated from crime scene evidence. In addition, some states have DNA profile databases contain- ing profiles from suspects and from unidentified human remains and missing persons. DNA profile databases have proven their value in many different situations. As of August 2018, use of CODIS databases had resulted in more than 400,000 profile matches that assisted criminal investigations and missing persons searches (Box 2). Despite the value of DNA profile databases, they remain a concern for many people who question the privacy and civil liberties of individuals versus the needs of the state. Technical and Ethical Issues Surrounding DNA Profiling Although DNA profiling is sensitive, accurate, and powerful, it is important to be aware of its limitations. One limitation is that most criminal cases have either no DNA evidence for analysis or DNA evidence that would not be informative to the case. In some cases, potentially valuable DNA evidence exists but remains unprocessed and backlogged. Another serious problem is that of human error. There are cases in which innocent people have been convicted of violent crimes based on DNA samples that had been inadvertently switched during processing. DNA evidence samples from crime scenes are often mixtures derived from any number of people present at the crime scene or even from people who were not present, but whose biological material (such as hair or saliva) was indirectly introduced to the site (Box 3). Crime scene evidence is often degraded, yielding partial DNA profiles that are difficult to interpret. One of the most disturbing problems with DNA profiling is its potential for deliberate tampering. DNA profile technologies are so sensitive that profiles can be generated from only a few cells—or even from fragments of synthetic DNA. There have been cases in which criminals have introduced biological material to crime scenes, in an attempt to affect forensic DNA profiles. It is also possible to manufacture artificial DNA fragments that match STR loci of a person’s DNA profile. In 2010, a research paper! reported methods for synthesizing DNA of a known STR profile, mixing the DNA with body fluids, and depositing the sample on crime scene items. When subjected to routine forensic analysis, these artificial samples generated perfect STR profiles. In the future, it may be necessary to develop methods to detect the presence of synthetic or cloned DNA in crime scene samples. It has been suggested that such detections could be done, based on the fact that natural DNA contains epigenetic markers such as methylation. Many of the ethical questions related to DNA profiling involve the collection and storage of biological samples and DNA profiles. Such questions deal with who should have their DNA profiles stored on a database and whether police should be able to collect DNA samples without a suspect’s knowledge or consent. Another ethical question involves the use of DNA profiles that partially match those of a suspect. There are cases in which investigators search for partial matches between the suspect’s DNA profile and other profiles in a DNA database. On the assumption that the two profiles arise from two genetically related individuals, law enforcement agencies pursue relatives of the person whose profile is stored in the DNA database. Testing in these cases is known as familial DNA testing. Should such searches be considered scientifically valid or even ethical? As described previously, it is now possible to predict some facial features and geographic ancestries of persons based on information in their DNA sample—a method known as DNA phenotyping. Should this type of information be used to identify or convict a suspect? As DNA profiling becomes more sophisticated and prevalent, we should carefully consider both the technical and ethical questions that surround this powerful new technology.