It's Evident


Advances in DNA
Alan Balfour, J.D., Ph.D., and Susan Zucker, Ph.D., Director Technology & Distance Education

Advances in DNA technology benefit law enforcement both in corroborating allegations and providing fresh intelligence. This article reports on recent advances that allow improved testing of old, degraded, or small samples of DNA.

Forensic applications of DNA analysis used to be conducted using a technique known as restriction fragment length polymorphism (RFLP). This technique was limited in application because it required relatively large sample sizes. In many instances, these sample sizes were not available. The technique was also often limited by sample degradation by environmental factors such as dirt or mold. Now, however, laboratories can develop profiles from biological evidence invisible to the naked eye, such as skin cells left on ligatures or weapons.1

Recent Advances

DNA analysis has been enhanced by the development of several techniques other than RFLP. One is PCR (polymerase chain reaction) testing. The PCR technique enabled laboratories to develop DNA profiles from extremely small samples of biological evidence. Exact copies of DNA contained in a biological evidence sample are replicated using the PCR technique. The process can reproduce millions of copies of DNA contained in a few skin cells. This technique does not affect the original. Since PCR analysis requires only a minute quantity of DNA, it can enable the laboratory to analyze highly degraded evidence for DNA. The downside of the sensitive PCR technique is that it replicates all DNA contained in an evidence sample increasing the likelihood of contamination in identifying, collecting, and preserving DNA evidence. In cases where evidence may have been improperly collected or stored, these factors may be particularly important.2

Short Tandem Repeat (STR), which evaluates specific regions (loci) found on nuclear DNA, reduces some of these restrictions. The variable (polymorphic) nature of the STR regions analyzed for forensic testing intensifies the discrimination between DNA profiles. For example, the likelihood that any two individuals (except identical twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion or greater.3

The FBI has chosen 13 specific STR loci to serve as the standard for the Combined DNA Index System (CODIS), which is a collection of databases of DNA profiles obtained from evidence samples from unsolved crimes and from known individuals convicted of particular crimes. Contributions to this database are made through State crime laboratories and the data are maintained by the FBI. The purpose of establishing a core set of STR loci is to ensure that all forensic laboratories can establish uniform DNA databases and, more importantly, share valuable forensic information. If the forensic or convicted offender CODIS index is to be used in the investigative stages of unsolved cases, DNA profiles must be generated by using STR technology and the specific 13 core STR loci selected by the FBI.4

In cases where DNA profiles from evidence are not suitable for RFLP or STR analysis there are two other types of DNA analysis which may be used. One is Mitochondrial DNA (mtDNA) which analyzes DNA found in a different part of the cell --- the mitochondrian. The other is Y-chromosome analysis.5

mtDNA technology is especially useful in examining old remains and evidence lacking nucleated cells — such as hair shafts, bones, and teeth — that are not amenable to STR and RFLP. mtDNA testing can be very valuable to the investigation of an unsolved case. For example, a cold case log may show biological evidence in the form of blood, semen, and hair was collected in a particular case, but that all were improperly stored for a long period of time. Although PCR analysis sometimes enables the crime laboratory to generate a DNA profile from very degraded evidence, it is possible that the blood and semen would be so highly degraded that nuclear DNA analysis would not yield a DNA profile. However, the hair shaft could be subjected to mtDNA analysis and be the key to solving the case.6

All maternal relatives have identical mtDNA. This enables unidentified remains to be analyzed and compared to the mtDNA profile of any maternal relative for the purpose of aiding missing persons or unidentified remains investigations. Both mtDNA and STR are excellent techniques used in the investigation of old cases.7

Several genetic markers have been identified on the Y chromosome that can be used in forensic applications. Y chromosome markers target only the male fraction of a biological sample making this technique quite valuable if the laboratory detects complex mixtures (multiple male contributors) within a biological evidence sample. Contrasted to the mtDNA technique, the Y chromosome is transmitted directly from a father to all of his sons, so it can also be used to trace family relationships among males. It is expected that the need for laboratories to extract and separate semen and vaginal cells (i.e., a vaginal swab or a rape kit) prior to analysis will be eliminated as Y-chromosome testing becomes more advanced.8

Deciding which analytical methods will be most valuable in a particular case requires cooperative efforts with the crime laboratory. As previously mentioned above, current DNA databases are being populated with DNA profiles generated using STR analysis rather than RFLP and mtDNA testing as the latter two are not compatible with the convicted offender or forensic indexes of CODIS. However, CODIS has a missing persons index that exclusively contains mtDNA profiles; the convicted offender and forensic indexes of CODIS exclusively contain STR DNA profiles.9

The increased acuity of these technology breakthroughs permit investigation of small samples of evidence from previously unsolved cases. Valuable DNA evidence that previously went undetected may now prove useful. Further, tests are faster and the matching easier because there has been a significant development in databanks at all levels of government – local, state and federal.


All fifty states collect DNA samples from convicted sex offenders. Some states also collect samples from those convicted of violent crimes. The Florida Department of Law Enforcement (FDLE) is requesting the state legislature to add convicted burglars to the list of those requiring DNA samples. This is because, according to the FDLE, 54% of people found guilty of burglary will eventually be found guilty of a murder or sexual assault.10

Databases at all levels of government have greatly enhanced law enforcement's ability to solve cases with DNA. Convicted offender databases allow law enforcement officers to compare crime scene evidence with thousands of potential suspect DNA profiles.

CODIS, the largest national databank, is a network of databases which is populated with data via state crime laboratories. Because many crimes have a high recidivism rate, an individual who committed a crime being investigated easily could have been convicted of a similar crime previously. In those cases, there is a DNA profile already in CODIS. CODIS features the ability to run a cross-comparison of DNA profiles and biological evidence found at crimes.11 Even if a perpetrator is not identified through the database, crimes may be linked to each other, thereby aiding an investigation, which may eventually lead to the identification of a suspect.12

This capacity facilitates law enforcement in their efforts to find matches of evidence even when no suspect has been identified. This has resulted in a reversal of traditional roles. As one laboratory expert said, “The investigators used to phone me with names. Now I phone the investigators.”13

The new processes are also valuable in solving cold cases. “If previously tested biological evidence produced a DNA profile but excluded the original suspect, revisiting those “exclusion” cases in the context of comparing them with DNA databases might prove to be very valuable to solving cold cases.”14

DNA Used to Exonerate

DNA evidence is not only used to convict; it is also used to exonerate. To date, the Innocence Project, a national litigation and public policy organization dedicated to exonerating wrongfully convicted individuals through DNA testing and reforming the criminal justice system to prevent future injustice,15 has exonerated 272 people based on DNA evidence. Given the number of cases affected by DNA evidence and the clear social and moral consequence involved, some states have formulated Post Conviction Plans which take a second look at cases where DNA evidence still exists and could be pivotal to a determination of guilt.16

DNA Controversy

There is no controversy in the scientific community about the validity of DNA profiling, but there is plenty of controversy concerning how this evidence is used in courts.17 Jurors in the O.J. Simpson trial apparently chose to ignore such evidence while it is speculated that its absence in the Casey Anthony trial led jurors to find her not guilty. Some of the lack of consistency may be attributable to jurors having heightened expectations of its use – the “CSI Effect”. Offsetting this elevated expectation is the apparent lack of ability of some jurors to understand expert scientific testimony as to the meaning of results. Hopefully, the recent advances discussed will result in not only greater scientific understanding but also more appropriate application in the courtroom.


Utilizing these scientific advances requires coordinated efforts between law enforcement personnel and crime laboratories. Issues of concern include logistics, cost, and the discriminating power of the tests. Lab experts aid law enforcement personnel by providing insight on which evidence might yield valuable results. Good cooperation between laboratories and law enforcement personnel, increasing experience in entering DNA evidence in courtrooms, and the ever-advancing nature of science are expected to provide more justified convictions and to reduce mistaken ones in the future.

1  National Institute of Justice (NIJ) at Last checked on July 15, 2011.
2  Ibid.
3  Ibid.
4  Ibid.
5  Ibid.
6  Ibid.
7  Ibid.
8  Ibid.
9  Ibid.
10  Johns, C.J., Supra.
11  National Institute of Justice (NIJ), Supra.
12  Ibid.
13  Johns, C.J., Supra.
14  National Institute of Justice (NIJ), Supra.
15  Actual Innocence Project at: Last checked on July 15, 2011.
16  Johns, C.J., Supra.
17  Ibid.