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

DNA molecule

Copy DNA Copy intact DNA

Normal DNA signal

Degraded DNA Can’t copy broken DNA

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

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

Degraded DNA signal

Degraded DNA Mixture Victim DNA Degraded other DNA

Computer Interpretation of Quantitative DNA Evidence Commonwealth v. Lyons June, 2011 Reading, PA Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Cybergenetics © 2003-2011 Cybergenetics © 2003-2011

DNA evidence interpretation Lab Infer Evidence item Evidence data Evidence genotype 10, 12 @ 50% 11, 12 @ 30% 12, 12 @ 20% 10 11 12 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

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

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.

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

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).

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)

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.

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.

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