Working with DNA Experts and Evidence: A Conversation with Jenifer
Jenifer Smith and Luann Battersby
Dr. Smith -- Professor of Practice at Penn State’s School of Forensic Science
Ms. Battersby – Former DNA laboratorian and attorney using expert witnesses
What is the testing accuracy of DNA evidence?
Use of DNA testing – RFLPs, STRs and PCR?
What human sources produce useable DNA evidence?
Researching results: DNA databases available to law enforcement
Reliability -Low Copy Number (LCN) DNA – are these spots/stains/touched paper/
gloves/hair/cigarette results reliable? 100pg with PCR enough?
PCR and buccal (cheek) samples
What about the lab? (Accreditation)
What about the individual conducting the testing? Is proficiency
information available? What are typical credentials? Does the technician
testify?
Preferred software programs - GeneScan and Genotyper
On the Defense -Low copy DNA (touch DNA) – stochastic effects cause PCR reaction to be
problematic. Is it interpretable?
Reviewing data – complete? Are there too many PCR cycles? Is 28 too many? Is
40 too many? How can I tell?
Analytical threshholds: Fairness – even ‘too many’ cycles should not be ignored
if exclusionary.
How important do you think non-DNA evidence is to define the pool of suspects?
Outline -1-
Quantity and quality of DNA required -- chain-of-custody precautions
Common lab errors --ensuring confidence in results
Sampling techniques
Number of loci tested to ensure accuracy of identification in PA
Labeling
Testing procedures – reagent blanks, negative and positive controls
Prevention of tampering
Double-checking to prevent human error
Protection from cross-contamination
Actual implementer credentials
Some high-profile DNA newsmakers
The Innocence Project
FBI and laboratory credibility issue (hair/negative controls)
Planted/moved evidence (OJ case)
Outline -2-
10 Points on Using Expert Witnesses
1. Know and communicate what the expert needs to clarify
2. Use the appropriate expert
3. Research and know the expert’s background
a. Professional
b. Personal
4. Conflicts
a. PACER
b. Publication search (e.g., BNA/Bloomberg Law databases)
5. Meet the person
6. Have them review the facts of the case – rehearse and repeat!
7. Listen to the expert’s telling
a. It should explain where the fault lies
b. Beware of technical language
c. Does it include knowledge of the factors involved in the case
d. How does the expert respond to questions regarding what he or she
does not know
8. Consider that an expert can do additional research and that using outside
sources and hearsay is allowable in order to gather, cull and synthesize a
final opinion
9. An expert may be used to elicit a fact
10. Costs - Will a consulting expert versus a testifying expert be sufficient
Using experts -1-
For example: Using expert witnesses for DNA analysis
Experts can:
Explain safeguards and methodology
Explain plateauing or other technical issues
Help deciders to know whether the results are valid
Attacking the expert’s conclusions and finding flaws:
Quality of the lab
Extrapolation of results
Review of possible lab errors
Chain of custody and scene contamination
Using experts -2-
20 DNA Terms - You’re the Expert
1. Adventitious match - a match to the profile of a person that was not the true donor
2. Allele - different forms of a gene; in DNA profiling, it refers to different forms of an intron
3. Allelic dropout - the allele cannot be visualized (Absence of evidence is not evidence of absence)
4. Bases - primary units of DNA: A, T, C and G
5. Billion-1,000,000,000; Nanogram (ng) = 10-9 = flip of a billion; Picogram (pg) = 10-12 = 1000 ng
6. CODIS - Combined DNA Index System (FBI-based)
7. DNA fingerprinting/DNA profiling - visualized fragments from digestion of DNA that look like
barcodes
8. Electrophoresis - separation of DNA fragments by size
9. Exon versus Intron = coding DNA versus non-coding DNA. Introns are specific to individuals
10. Gene - a functional sequence of DNA
11. Genome – one’s entire (haploid or single copy) DNA complement or chromosomes
12. LCN = low copy number. Few DNA molecules are available in the sample: hair, debris under
fingernails, epithelial cells, touched paper, gloves
13. Loci/locus - a position on the genome
14.
PCR and nested PCR - polymerase chain reaction is a process of amplification of DNA to expand
the interpretable amount of DNA (nested PCR uses product from the first PCR reaction to further
expand and in the second reaction)
15. Polymorphic - a locus with many different alleles which helps differentiate between individuals
16. Probe - a tagged DNA fragment that allows visualization of other DNA fragments after
electrophoresis by binding to them
17. RFLP - restriction fragment length polymorphism is cut DNA with mostly long DNA intron
sequences
18. STR – short tandem repeats (microsatellites) of 2-5 DNA base pairs, which can be amplified for
identification by PCR, using unique sequences of flanking regions as primers to start the process
19. Stuttering-miscopying of the DNA template by the PCR reaction; test accuracy issue
20. Transition and transversion - mutation from one DNA base to another; Translocation - a copy of a
gene has been inserted somewhere else on the genome