DNA is present in nearly every cell of our bodies, and we leave cells behind everywhere we go without even realizing it. Flakes of skin, drops of blood, hair, and saliva all contain DNA that can be used to identify us. In fact, the study of forensics, commonly used by police departments and prosecutors around the world, frequently relies upon these small bits of shed DNA to link criminals to the crimes they commit. This fascinating science is often portrayed on popular television shows as a simple, exact, and infallible method of finding a perpetrator and bringing him or her to justice. In truth, however, teasing out a DNA fingerprint and determining the likelihood of a match between a suspect and a crime scene is a complicated process that relies upon probability to a greater extent than most people realize. Government-administered DNA databases, such as the Combined DNA Index System (CODIS), do help speed the process, but they also bring to light complex ethical issues involving the rights of victims and suspects alike. Thus, understanding the ways in which DNA evidence is obtained and analyzed, what this evidence can tell investigators, and how this evidence is used within the legal system is critical to appreciating the true ethical and legal impact of forensic genetics.

Although the overwhelming majority of the human genome is identical across all individuals, there are regions of variation. This variation can occur anywhere in the genome, including areas that are not known to code for proteins. Investigation into these noncoding regions reveals repeated units of DNA that vary in length among individuals. Scientists have found that one particular type of repeat, known as a short tandem repeat (STR), is relatively easily measured and compared between different individuals. In fact, the Federal Bureau of Investigation (FBI) has identified 13 core STR loci that are now routinely used in the identification of individuals in the United States, and Interpol has identified 10 standard loci for the United Kingdom and Europe. Nine STR loci have also been identified for Indian populations.

As its name implies, an STR contains repeating units of a short (typically three- to four-nucleotide) DNA sequence. The number of repeats within an STR is referred to as an allele. For instance, the STR known as D7S820, found on chromosome 7, contains between 5 and 16 repeats of GATA. Therefore, there are 12 different alleles possible for the D7S820 STR. An individual with D7S820 alleles 10 and 15, for example, would have inherited a copy of D7S820 with 10 GATA repeats from one parent, and a copy of D7S820 with 15 GATA repeats from his or her other parent. Because there 12 different alleles for this STR, there are therefore 78 different possible genotypes, or pairs of alleles. Specifically, there are 12 homozygotes, in which the same allele is received from each parent, as well as 66 heterozygotes, in which the two alleles are different.

DNA evidence is used in court almost routinely to connect suspects to crime scenes, to exonerate people who were wrongly convicted, and to establish or exclude paternity. DNA data is considered to be more reliable than many other kinds of crime scene evidence. For this reason, tissue samples are frequently collected by law enforcement officials from those individuals who are implicated (even loosely) in a crime. The unique profile of each DNA sample is analyzed for comparison to crime scene evidence. The DNA profile can also be stored in a database to compare with crime scene evidence from past and future crimes.

But under what circumstances should an individual be compelled to provide a sample for a DNA database? Originally, statutes mandating collection of tissue for DNA typing applied only to those people convicted of sex crimes or murder. This was due to the fact that there was usually an abundance of DNA at the scene of a rape or murder to compare to a suspect's DNA. More recent DNA collection laws have applied to all convicted felons, reflecting advances in DNA technologies that allow sufficient DNA samples to be obtained from scenes of more common crimes, such as burglary (Figure 1).

As of July 2008, all 50 states in the U.S. have laws requiring convicted sex offenders to submit DNA samples, and 46 states have laws that require other types of convicted felons to do so. Additionally, 11 states require DNA samples from those convicted of certain misdemeanors, and 12 states have laws authorizing DNA sampling from arrestees of certain types of crime, usually felonies (National Conference of State Legislatures, n.d.). In comparison, in the United Kingdom, all arrestees, regardless of the degree of the charge, can be compelled to provide a sample. In fact, even those who are merely suspects in a crime can be forced to offer samples (Oak Ridge National Laboratory, n.d.).

Those people opposed to DNA banking for law enforcement purposes note that arrestees are often found innocent of crimes. Retention of an innocent person's DNA can be seen as an intrusion of personal privacy and a violation of civil liberties. It is interesting to note that in the United States, under any other circumstance, the provision of a DNA sample would require informed consent and other protections for the donor. In contrast, an arrestee's DNA profile, once entered into a database, can be accessed by police, forensic scientists, or researchers without the consent of the donor. Another problem with the DNA database system is an exacerbation of the ethnic bias already present in the criminal justice system. If people from one ethnic group are more often arrested, tried, and convicted of felonies, they will be overrepresented in the database, potentially leading to even more arrests within that ethnic group.

Proponents of DNA databanks argue that major crimes often involve people who have also committed other offenses. Having DNA banked could potentially make it easier to identify suspects. It could also significantly cut down the cost of an investigation if an automated computer search could eliminate suspects or link a suspect to a crime scene

The current DNA database maintained by the FBI, known as the Combined DNA Index System (CODIS), contains case samples (DNA samples from crime scenes or "rape kits") and individuals' samples (collected from convicted felons or arrestees) that are compared automatically by the system's software as new samples are entered. As of February 2007, CODIS had produced over 45,400 "hits," which assisted in more than 46,300 investigations (Federal Bureau of Investigation, n.d.). However, contrary to how DNA analysis is portrayed on popular television shows, DNA samples are not analyzed within the course of an hour. Rather, the U.S. currently has an enormous backlog of samples waiting to be typed and entered into the database. Some of these samples are from cases that have outlasted their statutes of limitation, so even if these samples could help solve a crime, the crime can no longer be tried.

This delay brings up the dilemma of the validity of statutes of limitation. These statutes were established at a time when large quantities of physical evidence were required to match a suspect to a sample and when extended time periods significantly decreased law enforcement's ability to find a match, as well as the likelihood of successful prosecution. With the advent of DNA databanks and the possibility of storing samples indefinitely, the very notion of a statute of limitation now seems extremely outdated.

Of course, there are many other debatable issues concerning DNA banking. For instance, should the original tissue sample be stored indefinitely after the DNA profile has been entered into the database? Detractors note threats to genetic privacy, but proponents argue that future DNA typing methods will undoubtedly be developed and that old samples might have to be reanalyzed using new techniques. Also at issue is the reopening of old cases on the basis of new (DNA-based) evidence. Which cases should be eligible for reanalysis in light of this new evidence? Can equitable rules be established to allow reexamination of cases that were analyzed with less powerful lab techniques? Further public awareness of the power of DNA forensic technology will help lawmakers decide these issues in a way that seeks to strike a balance between protecting individuals' genetic privacy and protecting innocent citizens from crime.

DNA evidence is easy to obtain because genetic material is found in all human cells, save red blood cells. As a result, when we leave behind small biological bits of ourselves, these bits can be used to identify us and link us to the places we've been. With modern technology, the amount of DNA required for analysis can be obtained from even a miniscule biological sample, which allows police to match crime scene evidence with suspects. However, because forensics is a science largely rooted in probabilities, even a confirmed "match" does not supply concrete proof of guilt. In addition, DNA databases designed to simplify the process of connecting past offenders to recent crimes are fraught with concerns involving individual genetic rights, as well as problems related to delayed sample entry, both of which hinder the ultimate usefulness of these databases. As a result, even though forensics is undeniably important to the modern justice system, its personal ramifications and ethical questions are topics of continuing discussion within the scientific, law enforcement, and legal communities.