Science and the Law: the Implications of DNA Profiling

by Katie Lachter

Attorney General Janet Reno has described our system of justice as a search for the truth.(1) Increasingly, the forensic use of DNA technology is an important ally in that search. DNA fingerprinting, better known in the scientific realm as DNA profiling, has given police and the courts a means of identifying the perpetrators of rapes and murders with a very high degree of confidence. However, nine years after its introduction, forensic DNA typing is still used only selectively. This is due to a variety of factors, including the unavailability of forensic typing to local prosecutors, the time required to perform the typing, and the costs of the tests if private laboratories are utilized. Formerly used only in research labs, DNA fingerprinting has recently made headlines in the media due to the spectacle of the O.J. Simpson murder trial, where DNA tests played a crucial role. While DNA profiling is being used more and more to help convict as well as exonerate potential suspects in criminal cases, the jury is still out in terms of the role DNA will eventually play in the legal system. The broad ramifications of DNA fingerprinting, especially concerns over its misuse, have raised serious moral and ethical dilemmas.

The process of DNA Fingerprinting: How and Why It Works

DNA is the master molecule of life and controls the growth and development of every living thing. Except for identical twins, the sequence of the base pairs within the DNA helix is unique for every person, and forms the individual's genetic code or blueprint.(2) An individual's DNA remains the same throughout life, and it is a person's unique genetic code that allows scientists to identify an individual to the exclusion of all others. The DNA and hence the genetic code of humans is nearly the same for all individuals. It is the very small amount that differs from person to person that forensic scientists analyze to identify people. Sometimes, this can be a difference of just one base pair. These differences are called polymorphisms and are the key to DNA typing.(3)

There are many sources of DNA for testing. Blood is one of the key sources, though the surface on which a bloodstain is found can profoundly affect the ability to successfully perform an analysis. In addition, bloodstains may be mixtures of blood from two different people and can produce DNA profiles that are more complex than those from a single individual. Ironically, DNA profiling may be the only way to determine if a given stain is a mixture. Semen stains are the most common evidence to be submitted for DNA analysis, which is not surprising since the cases in which DNA testing has been used the most often are rapes. DNA can also be extracted from tissues (taken at autopsy), hair roots, saliva, and in rare instances, urine.(4) It is important to note that only a miniscule amount of DNA is needed for analysis. For example, the amount of DNA found at the root of one hair is usually sufficient. Environmental factors also play a role in determining whether a particular sample of DNA can be utilized. Moisture, sunlight, bacterial action and heat are detrimental to the DNA. Depending on the intensity and combination of these conditions, survival of the DNA is measured in weeks or months. Even so, DNA in usable amounts can often be recovered when no other test could be performed. DNA is particularly stable when dried. Once the sample is removed to a dry, cool, indoor environment, DNA survival in stains can be measured in years or even decades.(5)

Isolation of the DNA is the first of six steps in the process of DNA fingerprinting. Once the DNA has been separated, restriction enzymes are used to cut the DNA at specific places. The fragments of DNA are then separated by size through a process called gel electrophoresis. The DNA is loaded into a well at one end of a slab of semi-solid gel, a porous, Jello-like substance. When an electrical field is applied to the gel containing the DNA, the DNA moves toward the positive electrode because it has a negative electrical charge. The sieving action of the gel allows the small restriction fragments to migrate at a faster rate than the larger ones.(6) The separated fragments are then transferred to a nylon membrane, and some sequences are identified by interaction with a radioactive probe. Each probe typically hybridizes to only one or two specific places on the nylon sheet. X-ray film is exposed by the radioactive emissions of only the labeled sequences. The developed film, called an autorad, shows the pattern of a DNA profile.(7)

The process of determining a genetic profile using DNA fragments is called Restriction Fragment Length Polymorphism (RFLP) analysis. To eliminate any possibility of a mistaken identity, analysts use several different probes to look at several different DNA fragment patterns in a sample. More than one person might have a particular RFLP pattern, but it becomes less likely that multiple people will have two or more sequences in common. Statisticians call this the multiplication rule, because the individual probabilities of a mistaken identity for each RFLP pattern are multiplied together to find the overall probability.(8)

DNA fingerprints have been used in many areas of human health care research, as well as in the justice system. DNA fingerprinting is used to diagnose inherited disorders in both prenatal and newborn babies in hospitals all over the world. These disorders include cystic fibrosis, hemophilia, Huntington's disease, familial Alzheimer's, sickle cell anemia, and many others. Early detection of such disorders enables the medical staff to prepare themselves and the parents for proper treatment of the child. In some programs, genetic counselors use DNA fingerprint information to help prospective parents understand the risk of having a disabled child. DNA fingerprints are also vital to developing cures for inherited disorders. Research programs to locate the genes causing inherited disorders depend on the information contained in the profiles.(9)

In addition to using DNA fingerprints to link suspects to biological evidence, DNA fingerprints have other important uses in the court system. They have been used to establish paternity in custody and child support litigation. Parentage testing cases are numerically the largest user of DNA testing.(10) Most paternity testing is done for financial reasons, i.e., to establish legal responsibility and provide for support. But perhaps even more important are the emotional and social issues. When testing can demonstrate conclusively to a man that he is the father of a child, he is more likely not only to provide financial support, but emotional support as well. He is more likely to bond with the child and take an active role in its life.(11) Yet another use for DNA testing is settling immigration questions. In 1988, the United Kingdom Home office and Foreign and Commonwealth office ratified the use of DNA fingerprinting for the resolution of immigration contentions which hinge upon disputed family relationships.(12) The numerous applications of DNA fingerprinting has made it an invaluable resource in the forensic field.

The People V. Orenthal James Simpson

According to Time magazine, the announcement of the verdict in the O.J. Simpson murder trial on October 2, 1995, was "the single most suspenseful moment in television history."(13) At no time in history has someone as well-known and well-liked as O.J. Simpson been accused of such a heinous crime as slitting his ex-wife, Nicole Brown Simpson's, throat and knifing to death her friend, Ronald Goldman. The Simpson case was a riveting spectacle for the American public, obsessed as it is with celebrity. As part of the media frenzy, DNA profiling, the crucial evidence in the case, received unprecedented attention.(14) However, it is important to note that the existence of DNA evidence, as we've seen, does not necessarily lead to a conviction. The circumstances of a case are the most important factors in determining guilt or innocence, regardless of DNA. Due to the sensationalizing in the media, the public perception was that the prosecution in the Simpson case had a mountain of evidence based on DNA, when in reality they had little more than a molehill.

The Science of Blood

The prosecution's case was based essentially on DNA evidence. One of the most important pieces of evidence was a bloody glove found at Simpson's Rockingham estate. This glove, allegedly Simpson's size, in a style he had worn, and a match for the one found at the crime scene, was said to contain fibers consistent with Goldman's shirt, Brown's and Goldman's hair, O.J.'s Ford Bronco, and limb hair from a black man. The blood was said to be a match for Goldman, Brown, and Simpson.(15) Other important pieces of evidence included the bloodstained socks found on Simpson's bedroom floor, which contained blood that was a DNA match for both Simpson and Brown, the blood found at the back gate of the crime scene, which was a DNA match for Simpson, and the blood in O.J.'s Bronco, which matched all three of them. In addition, DNA testing as well as conventional serology testing linked Simpson to the drops of blood near the victims at the crime scene.

It was an extremely powerful case. If all of the above evidence had been accepted as authentic by the jury, it would have led them to convict Simpson. The other side to this story turned out to be more important than the DNA evidence, and that is the way in which this evidence was collected. On deeper investigation, it was discovered that the prosecution's apparently solid mountain of evidence was not without its faults and crevices. It came to light that the prosecution and the Los Angeles Police Department had made serious mistakes in the handling of the DNA evidence during the early hours of the investigation. These included sending a trainee to collect blood samples; she had never before had primary responsibility for collecting blood evidence from a crime scene. Even more damning was the fact that LAPD Detective Vannatter carried around O.J. Simpson's blood in a vial in an unsealed envelope for three hours and went for a cup of coffee before booking it. Trial evidence allowed the defense to argue that 1.5 cc's of blood could not be accounted for by the prosecution. The defense suggested that this blood was planted by Vannatter, since the blood on the gate at the crime scene was not found during the initial investigation. This created a strong doubt in the minds of the jurors. In addition, under cross examination, LAPD criminalist Dennis Fung conceded to a litany of procedural errors.

Due to the blunders and poor handling of the DNA evidence by the prosecution, the defense experts were able to shoot holes in what would have been a solid case. Laboratory negligence is a big issue concerning the admissibility of DNA evidence, and the defense used that to its benefit. The defense also brought up the results of a 1987 test in which both Cellmark and Forensic Science Associates reported out false results.(16) They claimed that these test results pointed to a lab error rate of 2% (1 in 50 tests). In actuality, those tests were irrelevant because changes in protocols were made as a result of these tests and since then many thousands of tests have been successfully completed. Despite this, the defense's claim struck a chord with many who are wary of laboratory errors falsifying DNA test results. O.J. Simpson was not the only one on trial; so too was the future of DNA admissibility. Many scientists were hoping that the conviction of a celebrity murderer based on DNA profiling would open doors for DNA in the courtroom. Instead, despite the glaring amount of DNA evidence against Simpson, the defense was victorious and Simpson was acquitted. The future of DNA in the legal system remains uncertain.

Moral and Ethical Implications

One of the lasting effects of the O.J. Simpson case will likely be greater scrutiny by defense lawyers of the prosecution's forensic DNA evidence presented in criminal cases. In the Simpson case, the defense, in essence, put the crime laboratory on trial. The National Research Council (NRC) report, entitled DNA Technology in Forensic Science, states:

"There is no substantial dispute about the underlying [DNA] scientific principles. However, the adequacy of laboratory procedures and the competence of the experts who testify should remain open to inquiry." (17)

The quality and reliability of forensic laboratories is only one of the many concerns about DNA fingerprinting. In addition to the question of its admissibility in court, there are many moral and ethical issues involved. First, there is the issue of whether DNA evidence prevents defendants from getting fair trials. Defense attorney Robert Brower believes unequivocally that DNA evidence does threaten the constitutional right to a fair trial. "In rape cases, when the semen has been matched with the defendant's and the chance that it came from another person is 33 billion to 1, you don't need a jury."(18) Of course, the other side to this issue is that if DNA can provide conclusive evidence that a suspect committed a crime, some would say that it is an obstruction of justice and inherently unconstitutional not to present this evidence in court. Something else to consider is that few defendants have the resources available to them that O.J. Simpson had. Would another defendant have been able to refute the DNA evidence on grounds of negligence by the prosecution? The DNA experts that Simpson's defense team employed cost millions of dollars for their time. A person of lesser means might very well have been convicted by tainted evidence.

The most contentious issue is the matter of how to calculate statistical probability, the odds that a match between DNA found at the crime scene and DNA taken from the suspect could be the result of coincidence.(19) To find a match, crime labs look at several sites where the DNA is known to vary. If these sites match, there is an extremely high probability that the samples came from the same person. To quantify these findings, investigators calculate the frequency with which each variation occurs in the suspect's population group. The frequencies for each site are then multiplied together to arrive at a figure for the complete DNA profile.(20) Therein lies the problem- some population geneticists maintain that the frequencies of genetic markers in population sub-groups (based on race) could differ widely from the frequencies found in larger population groups. If this is so, then any estimates calculated using the widely accepted methods could be considerably off the mark.(21) The question then arises- if someone is convicted using data determined in this way, is that fair? How accurate do the results have to be before they should be admitted in court? Proponents of this theory insist that extensive and expensive population studies must be completed before reliable estimates could be introduced into the courtroom, even if this takes a decade or more. In the meantime, how many felons will go free if the evidence is not allowed? There are no easy answers to these questions. While the debate ensues, a temporary solution is in place. DNA is admitted or excluded on a case-by-case basis, determined by the quality of the tests performed on the DNA samples and the circumstances of the case.

DNA testing is not just an ethical, legal, or public policy issue; it is also a women's issue. Ninety percent of the victims of crimes involving DNA identification are committed against women. The tests are most useful in sex crimes, traditionally the toughest to solve and among the most under-reported. Only about half of the reported rapes result in arrests, and less then half of the men arrested are convicted. DNA has made it a lot harder for violent offenders to prey on women with impunity.(22)

A Question of Justice

DNA fingerprinting is an important tool in the search for justice. It provides prosecutors with a way to finger suspects with a high degree of certainty, and can exonerate others without the expense and suffering caused by a trial. Despite this, questions remain concerning whether DNA evidence is a threat to the right to a fair trial as granted by the United States Constitution. There are also concerns about the statistical probabilities involved with DNA profiling. No matter how small the chance might be that two people will have the same profile, can we convict on probability? O.J. Simpson was the exception because his defense attorneys were able to attack the DNA evidence presented by the prosecution. Most cases in which DNA evidence is presented end in conviction. The controversies over the constitutionality and conclusiveness of DNA evidence will most likely continue even as DNA becomes more of a force in the courtroom. We can better understand the future of DNA fingerprinting by looking at the history of digital fingerprinting.(23) A similar debate, with identical questions, took place a century ago. Could more than one person have the same print? Would investigators take care in gathering the evidence? Could they fake the evidence? Now as then, as Daniel Koshland, the editor of Science, has observed, "Caution is appropriate; unreasonable doubt is not."(24) Ultimately, complete acceptance of current DNA forensic technologies is inevitable, as they will continue to become even more powerful than they are now. In the meantime, we cannot wait for these methods to become admissible. Justice delayed is justice denied.


Edward Connors, Thomas Lundregan, Neal Miller, and Tom McEwen, Convicted by Juries, Exonerated by Science: Case Studies in the Use of DNA Evidence to Establish Innocence After Trial (Virginia: U.S. Department of Justice, 1996), iii.

Howard Coleman and Eric Swenson, DNA in the Courtroom (Seattle:GeneLex Press, 1994), 30.

David F. Betsch, "DNA Fingerprinting in Human Health and Society," Genentech's Access Excellence,, (June 1994).

"DNA Fingerprinting," Newton's Apple,, (October 1994).

T. Burke, G. Dolf, A.J. Jeffreys, and R. Wolf, eds., DNA Fingerprinting: Approaches and Applications (Basel: Birkhauser Verlag, 1991), 1.

Alan M. Dershowitz, Reasonable Doubts (New York: Simon and Schuster, 1996), 46.


1. Edward Connors, Thomas Lundregan, Neal Miller, and Tom McEwen, Convicted by Juries, Exonerated by Science: Case Studies in the Use of DNA Evidence to Establish Innocence After Trial (Virginia: U.S. Department of Justice, 1996), iii.

2. Howard Coleman and Eric Swenson, DNA in the Courtroom (Seattle: GeneLex Press, 1994), 30.

3. Ibid., 33.

4. Ibid., 20.

5. Ibid., 27.

6. Ibid., 37.

7. David F. Betsch, "DNA Fingerprinting in Human Health and Society," Genentech's Access Excellence,, (June 1994).

8. "DNA Fingerprinting," Newton's Apple,, (October 1994).

9. Betsch, 2.

10. Coleman and Swenson, 62.

11. Ibid., 62.

12. T. Burke, G. Dolf, A.J. Jeffreys, and R. Wolf, eds., DNA Fingerprinting: Approaches and Applications (Basel: Birkhauser Verlag, 1991), 1.

13. Alan M. Dershowitz, Reasonable Doubts (New York: Simon & Schuster, 1996), 46.

14. Coleman and Swenson, 93.

15. Dershowitz, 30.

16. Coleman and Swenson, 97.

17. Connors et al., 25.

18. Coleman and Swenson, 5.

19. Ibid., 9.

20. Ibid., 9.

21. Ibid., 10.

22. Ibid., 111.

23. Ibid., 111.

24. Ibid., 111.

Term Projects - selected essays


Paul Reed
Sun Nov 23 23:05:44 PST 1997
Linking to your thesis, if that's what it is, "Science and the Law: the Implications of DNA Profiling," in my new weekly feature article at: . Thanks!

howard gaspard
Fri Dec 14 10:23:21 CST 2001
i would like to know the legalities and accuracy of a Blood dna over buccal testing- where I am being forced to test from some one who i dont know

Adelaide Zeta
Sat Nov 6 08:23:35 UTC+1000 1999
Thank-you for helping me out with my Essay.

Rebecca Brown
Fri Aug 13 10:57:45 UTC+1000 1999
This was so helpful, thankyou very much.


This place helped me alot with my Common assesment task. I am doing Year 12 in Australia. Thanks for the help mate.

Richard M Leary
Fri May 14 11:52:08 UTC+0100 1999
Enjoyed this very much. (Scientific Development Officer)West Mids Police UK. RMLEARY@AOL.COM.

Eric Hortop
Thu Apr 16 21:07:44 1998
Nice to see a balanced view of the legal system's new toy. Congrats. I'll be using you as a source and a springboard in a philosophy essay.

Greg Williams
Wed Oct 11 17:41:50 CDT 2000
Sorry to bother you, but I was just woundering if there was any flaws or faults in the DNA prifiling process, I'm sophmore year at Fairfield high and if you could give me some info on this for my paper that would be much apprediated. Thank you very much Greg Williams-

Emma Bartlett
Mon Oct 9 20:03:46 UTC+0100 2000
Your essay is very informative, and will help me immensly with my coursework. Thanks!!

Mary Roberts
Mon Jul 10 11:52:00 GMT-0400 (Eastern Daylight Time) 2000
I think this essay was very informative. It provided me with enough information needed to fully understand DNA testing in trials.

James Hernandez II
Wed Mar 01 01:07:43 2000
Thanks for the imformation, along with the help with my essay

James Hernandez II
Wed Mar 01 01:07:43 2000
Thanks for the imformation, along with the help for my essay