Comparison Of Dna Typing Methods

Technologies used for performing forensic DNA analysis differ in their ability to differentiate two individuals and in the speed with which results can be obtained. The speed of analysis has dramatically improved for forensic DNA analysis. DNA testing that previously took 6 or 8 weeks can now be performed in a few hours.

The human identity testing community has used a variety of techniques including single-locus probe and multi-locus probe RFLP methods and more recently PCR (polymerase chain reaction)-based assays. Numerous advances have been made in the last 15 years in terms of sample processing speed and sensitivity. Instead of requiring large blood stains with well-preserved DNA, tiny amounts of sample, as little as a single cell in some cases, can yield a useful DNA profile.

The gamut of DNA typing technologies used over the past 15 years for human identity testing is compared in Figure 1.1. The various DNA markers have been divided into four quadrants based on their power of discrimination, i.e., their ability to discern the difference between individuals, and the speed at which

Figure 1.1 Comparison of DNA typing technologies. Forensic DNA markers are arbitrarily plotted in relationship to four quadrants defined by the power of discrimination for the genetic system used and the speed at which the analysis for that marker may be performed. Note that this diagram does not reflect the usefulness of these markers in terms of forensic cases.

Markers Used (Biology)


Power of Discrimination (Genetics)


Power of Discrimination (Genetics)


Multi-Locus Probes


Single-Locus Probes

Multiplex STRs




single STR






Speed of Analysis (Technology)

they can be analyzed. New and improved methods have developed over the years such that tests with a high degree of discrimination can now be performed in a few hours.

An ABO blood group determination, which was the first genetic tool used for distinguishing between individuals, can be performed in a few minutes but is not very informative. There are only four possible groups that are typed - A, B, AB, and O - and 40% of the population is type O. Thus, while the ABO blood groups are useful for excluding an individual from being the source of a crime scene sample, the test is not very useful when an inclusion has been made, especially if the sample is type O.

On the other extreme, multi-locus RFLP probes are highly variable between individuals but require a great deal of labor, time, and expertise to produce a DNA profile. Analysis of multi-locus probes (MLP) cannot be easily automated, a fact that makes them undesirable as the demand for processing large numbers of DNA samples has increased. Deciphering sample mixtures, which are common in forensic cases, is also a challenge with MLP RFLP methods, which is the primary reason that laboratories went to single-locus RFLP probes used in serial fashion.

The best solution including a high power of discrimination and a rapid analysis speed has been achieved with short tandem repeat (STR) DNA markers, shown in the upper right quadrant of Figure 1.1. Also because STRs by definition are short, they can be analyzed three or more at a time. Multiple STRs can be examined in the same DNA test, or 'multiplexed.' Multiplex STRs are valuable because they can produce highly discriminating results (Chapter 5) and can successfully measure sample mixtures and biological materials containing degraded DNA molecules (Chapter 7). In addition, the detection of multiplex STRs can be automated, which is an important benefit as demand for DNA testing increases.

It should be noted though that Figure 1.1 does not fully reflect the usefulness of these markers in terms of forensic cases. Mitochondrial DNA (mtDNA), which is shown in the quadrant with the lowest power of discrimination and longest sample processing time, can be very helpful in forensic cases involving severely degraded DNA samples or when associating maternally related individuals (Chapter 10). In many situations, multiple technologies may be used to help resolve an important case or identify victims of mass disasters, such as those from the World Trade Center collapse (Chapter 24).

Over the past 20 years, there has been a gradual evolution in adoption of the various DNA typing technologies shown in Figure 1.1. When early methods for DNA analysis are superseded by new technologies, there is usually some overlap as forensic laboratories implement the new technology. Validation of the new methods is crucial to maintaining high quality results (Chapter 16). Table 1.1 lists some of the major historical events in forensic DNA typing. The implementation of new methods by the FBI Laboratory has been listed in this historical timeline because the DNA casework protocols used by the FBI create an important trend within the United States and around the world.


This book contains a review of the steps involved in processing forensic DNA samples with STR markers. STRs are a smaller version of the VNTR sequences first described by Dr. Jeffreys. Samples obtained from crime scenes or paternity investigations are subjected to defined processes involving biology, technology, and genetics (Figure 1.2).

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