DNA Profiling

To understand how DNA profiling is used to identify a person, imagine a sample of blood collected at a crime scene that doesn't match the victim's blood, and is presumably from the unknown perpetrator. DNA from the blood is isolated and its set of STRs are analyzed. The results will be a list of the alleles found at each of the markers (for example, VWA-12, 13; TH01-6, 7, and so on), where the initial symbol is the abbreviation for the markers and the last two are the numbers of the alleles found in the sample for that marker. The full set of thirteen markers may or may not be analyzed in each case. When a suspect is identified, his or her DNA can be analyzed for these same markers. If the set of alleles are different, the investigators can be sure that the two DNAs came from different sources, and the suspect is not the source of the blood. Since the introduction of DNA profiling, an absence of matching DNA has been used to free dozens of wrongly convicted prisoners.

If the samples do match, the question becomes whether the blood is actually from the suspect, or from someone else with the same set of alleles. As with blood typing, this is a matter of statistics, and depends on how frequently each allele occurs in the population. This information has been tabulated and is kept on file in the FBI CODIS database. If two samples share a very rare allele, that increases the likelihood they came from the same source.

Matching multiple alleles increases the certainty they came from the same source. Since the thirteen STRs are inherited independently of each other, the likelihood that one person's DNA will include specific alleles of all thirteen STR sites is the product of the individual allele frequencies. For example, if each allele a person carries occurs in 25 percent of the population, then the probability that all thirteen alleles will occur in one Locations of the FBI's 13 core loci used for DNA profiling. The loci were chosen to span the genome, and to have maximum variability among humans. Adapted from <http://www.cstl.nist.gov/biotech/strbase/images/codis.jpg>. individual is (0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25 × 0.25) or 1 in more than 67 million. This analysis can discriminate between millions of people, far better than is possible using the four blood groups. Since many alleles are even rarer than 25 percent, their presence in both samples further increases the probability that they came from the same source.