1. severed carotid artery
Commonwealth v. Lyons
homicide: DNA mixture evidence
75% victim
25% other
Victim's blood spatter
pulsating spray from
severed carotid artery

2. DNA molecule

3. Copy DNA
Copy intact DNA

4. Normal DNA signal

5. Degraded DNA
Can’t copy broken DNA

6. Longer molecules copy less
With degradation,
a longer DNA molecule has
a greater chance of having a break

7. DNA decay curve With degradation, a longer DNA molecule
makes fewer DNA copies
Observed DNA
DNA size

8. Degraded DNA signal

9. Degraded DNA Mixture
Victim DNA
Degraded other DNA

10. Computer Interpretation of Quantitative DNA Evidence
Commonwealth v. Lyons
June, 2011
Reading, PA
Mark W Perlin, PhD, MD, PhD
Cybergenetics, Pittsburgh, PA
Cybergenetics ©
Cybergenetics ©

11. DNA evidence interpretation
Lab
Infer
Evidence item
Evidence data
Evidence genotype
10, 50%
11, 30%
12, 20%
All DNA interpretation is done in the same general way. The crime lab processes DNA evidence items to form DNA evidence data, from which evidence genotypes are objectively inferred. When there is uncertainty, the allele pair genotype values have an associated probability. Following genotype inference, a comparison is made with a suspect (or other reference) genotype to form a DNA match statistic.
Compare
Known genotype
10, 12

12. Computers can use all the data
Quantitative peak heights at locus D8S1179
victim
Computers can use all the quantitative evidence data.
other

13. People may use less of the data
All-or-none allele peaks; ignore victim genotype
Over threshold,
peaks become
allele events.
Human review generally uses only some of the data, discarding quantitative information and knowledge of the victim genotype.
Threshold
Under threshold,
alleles vanish.

14. How the computer thinks
Consider every possible genotype solution
other
victim
The computer considers every possible genotype solution. Solutions that better explain the observed data are assigned a higher likelihood.
Explain the
peak pattern
Better explanation
gives a
higher probability

15. Evidence genotypes other victim
In this case at locus D18, the victim-matching genotype was inferred with certainty (14, 15), while there was some uncertainty in the other inferred genotype, with most of the probability concentrated around allele pair (13, 14).

16. How much more does the suspect match the evidence
DNA match information
How much more does the suspect match the evidence
than a random person? 6x
Probability(evidence match)
The DNA match statistic assesses the relative probability of an evidence match to a coincidental match.
Probability(coincidental match)

17. Is the suspect in the evidence?
A match between the suspect and the evidence
is 9.46 trillion times more probable
than coincidence.
Yes, with a reasonable degree of scientific certainty,
evidence item Q9 contains DNA
from the genotype of suspect item K2.
The evidence indicates that the suspect contributed to the evidence.

18. Is the victim in the evidence?
A match between the victim and the evidence
is 1.27 quintillion times more probable
than coincidence.
Yes, with a reasonable degree of scientific certainty,
evidence item Q9 contains DNA
from the genotype of victim item K1.
The evidence indicates that the victim contributed to the evidence.

19. Match statistic comparison
Computer
Human
9,500,000,000,000
42,000