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FORENSIC DNA PROFILING: Strengths and Limitations

BY

FATOKI, Toluwase Hezekiah

M.Tech Seminar (BCH 810)

FORENSIC DNA PROFILING: Introduction Forensic science is the application of scientific

knowledge and experimentation to legal contentions, be they civil or criminal matters which are enforced by police agencies.

Locard’s exchange principle: whenever two objects come into contact with one another, there is exchange of material between them.

Deoxyribonucleic acid (DNA)

DNA profiling (also known as DNA typing or DNA fingerprinting) is a forensic technique used in the identification individuals by characteristics of their DNA.

Polymerase Chain Reaction (PCR) by Kary Mullis at USA in 1983. Restriction Fragment Length Polymorphism (RFLP) analysis by Professor Sir Alec Jeffreys at UK in 1985.

Human Genome Project , completed in 2004 Synthetic Human Genome, proposed in June, 2016

(www.hgpwrite.org) © Lehninger Principles of Biochemistry (2008), 5th Ed. pg352

© Butler John M (2005). Forensic DNA Typing., Elsevier Academic Press, p18

FORENSIC DNA LABORATORY PROCESS A general flow for a forensic laboratory process.

Overall the steps involved in a forensic DNA laboratory can be separated into the pre-laboratory, laboratory, and post laboratory.

Before DNA can be isolated, it is important to verify the true identity of the forensic samples.

The efficiency of different DNA extraction methods is dependent upon the exact nature of the sample (such as blood, semen, saliva, urine, sweat, and bone etc.).

©Wyndham Forensic Group Inc. (2009).

©Wyndham Forensic Group Inc. (2009).

EXAMPLES OF DNA PROFILES AND INTERPRETATION DNA profiling essentially involves pattern matching.

Short Tandem Repeat markers visualized by silver staining (C) Examples of allelic patterns for 3 multiplex STR loci from amplified DNA. Slab Gel Electrophoresis.

Short Tandem Repeat markers visualized by fluorescent primers (D) Examples of allelic patterns for 2 individuals using 10 multiplex STR loci and a gender ID. Capillary Gel Electrophoresis. Multiplexing: Fluorescence-based automated sequencers.

©Kloosterman A. D.(2003). Current and Future Developments in Forensic DNA Typing. Nederlands Forensisch Instituut. ©Wyndham Forensic Group Inc. (2009).

TARGETS OF DNA ANALYSIS: Definition, Strength and limitation

Short Tandem Repeats (STR) Analysis:

STR are short segments single DNA strand consisting of 2-7repeating base pairs.

Most informative with samples that involve well-preserved soft tissue and bone.

Unsuitable for the analysis of highly degraded or low copy number DNA samples, and there

are inconsistency of number of STR markers in different DNA databases.

Single Nucleotide Polymorphism (SNP) Analysis:

DNA sequence variations that occur when a single nucleotide (A, T, C, or G) in the genome

sequence is altered.

SNPs offer an advantage over STRs due the fact that heavily degraded DNA fragments can be

analyzed with SNPs.

Less informative per locus than STR, and unsuitable for DNA mixtures

Romeika JM,Yan F (2013) Recent Advances in Forensic DNA Analysis. J Forensic Res S12: 001.

TARGETS OF DNA ANALYSIS: Definition, Strength and limitation (Contd)

Analysis of Degraded or Low Template (LT) DNA:

When less than 200 picogram of DNA is found in a sample.

LT or degraded DNA has been successfully amplified for STR genetic profiling using whole

genome amplification (WGA), so that it can be further analyzed in the PCR method.

Ease of contamination and amplification of contaminants, and mixed profiles being produced

and wrongful accusations.

Mitochondrial DNA (mtDNA) Analysis:

A circular molecule of DNA 16,569 base pairs in size, obtained from the mitochondrion

organelle found within cells.

MtDNA is often used in LT DNA due to the higher proportional amount of mtDNA to nuclear

DNA, and less prone to degradation.

Less discriminatory (maternal inheritance), relatively time consuming and expensive.

Romeika JM,Yan F (2013) Recent Advances in Forensic DNA Analysis. J Forensic Res S12: 001.

DNA Methylation Analysis:

DNA methylation is a biochemical process involving the addition of a methyl group to the

cytosine or adenine DNA nucleotides.

DNA methylation appears to be the best suited for body fluid identification presently, due to its

high specificity and compatibility with current STR typing protocols.

Large amount of samples needed, labor-intensive and time-consuming.

Genome Profiling (GP):

Used to distinguish between human and non-human DNA without the use of sequencing, PCR

and temperature gradient gel electrophoresis (TGGE) analysis.

Results within a shorter time frame, limited technical skill is needed, and no expensive

equipment is required.

The results could be unreliable if the samples are contaminated with other genomes.

TARGETS OF DNA ANALYSIS: Definition, Strength and limitation (Contd)

FUTURE OF FORENSIC DNA PROFILING: Definition, Strength and Limitation

Microfluidic Systems and Nanotechnology for DNA Analysis: Microtechnology and nanotechnology have the ability to create devices that can prepare,

manipulate, and analyze at a very small levels. Microdevices and nanoparticle allows minimal reagent and sample volume, decrease the

chances of contamination. Nanoparticles takes a role in the use of DNA biosensors. One of the drawbacks is that both often does not quantify the DNA.

DNA Databases: The databases can be used for intelligence. It contains the STR profiles of known offenders and

crime samples. A set of standard DNA markers have been determined as criteria for a DNA profiling system

called the Combined DNA index system (CODIS) in USA, and Second Generation Multiplex DNA profiling (SGM+) system in UK.

What about other countries? Check: www.genewatch.org/sub-566699.

Forensic DNA Phenotyping (FDP):

The new tool for finding unknown person in the absence of a universal DNA profile database. It includes the inference of information on biogeographic ancestry and externally visible

characteristics (EVCs) directly from a DNA sample FDP is not free of ethical issues and seem to have serious error rate

Kayser and de Knijff (2011). NATURE REVIEWS | Genetics, Vol 12; 179-192.

FORENSIC DNA ANALYSIS: The OverallSTRENGTHS

1. It can be extremely discriminating, due to low random probabilities.

2. It is extremely sensitive, at very low quantities of DNA.

3. Profile can be successfully developed from trace evidences, and detection of modifications.

4. Multiple testing systems are available.

5. Mixtures of DNA can be analysis.

LIMITATIONS

1. Coincidence can occur in random probability.

2. Low levels of DNA on various surfaces and objects, whether a crime has occurred or not. Sterilization does not guarantee DNA-free.

3. Searching evidence for sample on which to perform DNA testing is challenging. The process is low-tech, meticulous, labor-intensive and subject to error. Fake (synthetic) DNA

4. A failure to exclude someone using either mtDNA or Y-STR testing is different than when using standard STR testing. The Chimeras.

5. Low levels of DNA in mixture pose greater challenge and interpretation may carry a higher risk of error.

CONCLUSION Forensic DNA Profiling has had a tremendous positive impact in the criminal justice system,

but its reliability should not be taken for granted.

It is worth noting that although numerous scientific improvements are sure to come, the current

methods are reliable and valid for the present crime puzzles. However, with any DNA

techniques, the juror should not convict on DNA profiling evidence alone if other factors raise

doubts.

The great potential of automated microfluidic devices coupled with nanotechnology for high

throughput DNA analysis is yet to be fully harnesses. It is my expectation that the newly

proposed synthetic human genome project, when launched and completed will aid the strength

of this process in the future.

Fatoki T. Hezekiahhezekiahfatoki@gmail.com

www.linkedin.com/in/hezekiahfatoki www.facebook.com/groups/africaforensicwww.fheztbioinformatics.wordpress.com