Forensic Science in the Courtroom Final Exam

What does it mean to obtain a DNA match between a suspect and material from a crime scene?

is the fact that a DNA profile obtained mean that this information is probative?

LT-DNA refers to

the amount of DNA recovered from a sample

< 100 pg of DNA =

16 haploid cells

what factors must be considered when analyzing LCN DNA?

1. contamination risks

2. maximizing DNA recovery from evidence items

3. sampling error in the amplification tube

   1. stochastic effects

   2. preferential amplification of the DNA template amplified first

4. interpretation guideline development

stochastic effects

random effects

various modifications to work with LT-DNA

1. dedicated lab space

2. additional quality controls

3. sample purification and concentration

4. additional PCR cycles (31-34)

5. Enhancement of electrophoresis conditions

6. triplicate amplification

7. conservative interpretation

Conservative interpretation

consider robust vs. non-robust loci; amount input DNA

if you have a small amount of DNA,

you can add extra cycles at the end to double it

Why is triplicate amplification a problem?

because of the fact we do not know where some of the DNA is coming from

What degrades DNA?

UV irradiation

stratalinker

STR Kit QA

kits are tested upon receipt and every 4 months

used for lots of DNA

found by accident

positive controls

used to test the robustness of the assay

negative controls

used to identify possible contamination during testing

allow up to 9 non-repeating alleles across 3 test tubes (NYC)

if it is non coding,

there is no dominant and no recessive

PCR

a mechanism for copying over DNA

making copies of a targeted area of the gene

repeated cycles of heating and cooling, copying

cycling

about 30 cycles

amount of PCR template doubles each cycle

after n cycles, amount of PCR product, P is (2ⁿ)t

in theory, one template strand of DNA will yield over _________ copies after _____ cycles

67 million; 28

stutter

insertion caused by backward slippage

deletion caused by forward slippage

increased stutter

occurs more frequently

stutter occurring in an early cycle of PCR,

the product can be much larger than expected

Allelic Droupout

Results in false homozygote or both alleles may dropout

Amplified in Triplicate

increases. accuracy of allelic assignments

• identify drop-outs

• identify drop-ins

increases # of determined alleles and loci in a sample

possible outcomes of replication

1. sample interpreted as single source

2. mixture sample deconvoluted (major donor only)

3. mixture for comparison only

   1. no major donor

4. inconclusive sample

   1. too few or too many alleles to draw any conclusions

Primary Trasfer

subject 1 touches an object, subject 1’s cells are deposited

Secondary Transfer

subject 1 after contact with subject 2 touches an object: subject 2’s cells are deposited in addition to subject 1’s

the major component of the mixture of two people is ________

the primary DNA donor, 1

following a handshake, the DNA recovered from weapons handled immediately produces _________

mixtures

common courtroom challenges

1. forensic DNA analysts cannot determine when or how the sample was deposited

2. highly unlikely that one could attribute a DNA profile in a case to secondary or tertiary transfer

3. transfer arguments

4. not yielding the same results

5. discussions of drop in and drop out

6. exclusions

why 3 amplifications and not 4, 5, 6?

not enough DNA, can only be broken down so far

pre-testing strategies

1. LT-DNA testing will consume samples

2. Testing is extremely sensitive and mixtures are commonly detected

considerations for Court

1. educate attorneys on the limitations of LT-DNA testing

2. Explaining results to the court

3. Practice and training are needed to testify to these results