Home > Trayvon Martin > DNA TESTING IN THE TRAYVON MARTIN/GEORGE ZIMMERMAN CASE

DNA TESTING IN THE TRAYVON MARTIN/GEORGE ZIMMERMAN CASE

  

 

While reading articles on the DNA results in the Trayvon Martin/George Zimmerman case I soon realized that a lot of detail must have been released about the test results.  After a quick internet search I was somewhat shocked to discover that official DNA reports and even some laboratory notes are available online: http://media.cmgdigital.com/shared/news/documents/2012/09/19/case_part_5_Gorgone_FDLE_complete_report_7_26_12_.pdf

The media has made attempts at explaining and interpreting the findings, but of course DNA reports are not necessarily intuitive for laypersons.  This is a rare opportunity in which an official DNA report is available for public discussion, so I thought I would weigh in and hopefully add further depth of analysis and perspective on the DNA findings.

I will offer my standard disclaimers:  I am not an official expert on this case, I do not have the entire discovery that I would need to perform an official review, etc.  Unfortunately the laboratory notes only pertain to a few samples, so I am left basically with just the final reports for commentary, and am taking them at face value for purposes of this discussion.

Florida Department of Law Enforcement Report Dated March 26, 2012

Exhibit DMS-21 A:  Swab from grip of gun

“gave chemical indications for the presence of blood” 

–This sounds as if a screening test for blood was performed and was positive.  A screening test alone can only indicate that blood (not necessarily human) may be present, but it is not considered a confirmatory test.  Screening tests are used due to their sensitivity, but they are not specific, meaning other substances can give positive results.  Additional testing is typically required to establish that human blood is present.  Appearance of the staining and detection of human DNA are also factors that may strengthen the hypothesis that human blood is present.  If suspected bloodstains are limited in size, there may not be enough material to subject to a confirmatory test for human blood, without jeopardizing the ability to obtain DNA results.

So, “gave chemical indications for the presence of blood” in and of itself would translate to “the possible presence of blood was detected”.

The mixed DNA profile obtained from Exhibit DMS-21A demonstrated the presence of at least two individuals. A complete DNA profile for the major contributor could be determined.” 

–DNA testing can detect mixtures of DNA from multiple individuals; however it is not possible to state with certainty the exact number of contributors there are to a DNA mixture.  Therefore, the verbiage “at least” is applied to the number of contributors in the DNA conclusion.

The major contributor in a DNA mixture refers to the individual contributing the highest amount of DNA in a mixture.  In this instance, a DNA profile clearly belonging to the major contributor could be deciphered from the mixture and compared to known DNA profiles.

This major DNA profile is consistent with originating from a male individual and matches the DNA profile from George Michael Zimmerman (JR-2).”

–The term ‘match’ in and of itself is insufficient to describe the significance of the DNA profile being ‘consistent’ with Zimmerman.  However, at the end of the conclusion, statistics are provided which illustrate that the DNA profile is likely either from Zimmerman or an identical twin of Zimmerman.

A DNA profile for the minor contributor(s) could not be determined” 

–This sounds as if there was very little information about the individual(s) who were present in lesser quantity than the major contributor, however we do know that:

Trayvon Benjamin Martin (ME-3) is excluded as a possible contributor to the mixed DNA obtained from Exhibit DMS-21A” 

In summary, blood may be present, and DNA from Zimmerman and at least one other individual (not Martin) is present.

Exhibit DMS-21 B:  Swab from trigger of gun

It appears that a screening test for blood was not performed.

Due to the limited nature of the DNA results obtained from Exhibit DMS-21B, the data obtained is not interpretable” 

–It sounds like a very low level of DNA was recovered, and provided very little information.

Exhibit DMS-21C:  Swab from slide of gun 

“failed to give chemical indications for the presence of blood” 

–A negative screening test for blood is very strong evidence that blood is not present, due to the sensitivity of screening tests.

“Due to the limited DNA results obtained from Exhibit DMS-21C, this data is insufficient for inclusion purposes.  Results are consistent with the presence of at least one male individual.” 

–Some laboratories will use weak DNA data to exclude individuals, but will judge that it is not adequate to include individuals.  In short, this is because with low levels of DNA, portions of DNA profiles may not show up in testing.

Often if low-level partial information is detected it can be difficult to determine if the results represent a small portion of a DNA profile from one individual or portions of DNA profiles from multiple individuals.  In these cases, the analyst essentially has to ‘punt’.

“No determination can be made regarding the possible contribution of George Michael Zimmerman (JR-2) or Trayvon Benjamin Martin (ME-3) to the limited DNA results obtained from Exhibit DMS-21C” 

–Translation…can’t rule either Zimmerman or Martin in and can’t rule them out. There is not enough data to conduct a meaningful comparison. In my experience, sometimes analysts are too quick to ‘punt’.  Another question that can be answered sometimes even when data is limited is “was there any DNA present that could not have come from Zimmerman or Martin, even if in very low amount?”  Without knowing the actual DNA types detected, I cannot answer that question and do not know if it was considered by the analyst.

In summary, blood was not detected, and while some DNA was detected it was judged insufficient to determine whether Zimmerman and Martin were included or excluded as possible contributors.  It is unclear if there is any DNA that could not have come from Zimmerman or Martin.

 Exhibit DMS-21D:  Swab collected from holster

“failed to give chemical indications for the presence of blood” 

–A negative screening test for blood is very strong evidence that blood is not present, due to the sensitivity of screening tests.

The mixed DNA profile obtained from Exhibit DMS-21D demonstrated the presence of at least three individuals.  A complete DNA profile for the major contributor could be determined.” 

–The major profile matched Zimmerman with strong statistical significance. This sample has at least 2 minor contributors in addition to the DNA profile matching Zimmerman.

“A DNA profile for the minor contributor(s) could not be determined” 

“No determination can be made regarding the possible contribution of Trayvon Benjamin Martin (ME-3) to the mixed DNA profile obtained from Exhibit DMS-21D. 

–Trayvon Martin could not be included or excluded.  The more possible contributors present in a mixture, the more difficult the interpretation becomes.  When the levels of DNA are low, the interpretation is further complicated.

Exhibit ME-2:  Fingernail scrapings represented as being from Trayvon Benjamin Martin

“gave chemical indications for the presence of blood”

Right hand:  “No DNA results foreign to Trayvon Benjamin Martin (ME-3) were found on Exhibit ME-2A”

Left hand:  “No DNA results were obtained from Exhibit ME-2B

–Self-explanatory

Exhibit ME-8:  Shirt represented as being from Trayvon Benjamin Martin

Stain A 

“gave chemical indications for the presence of blood”

“This DNA profile is consistent with originating from a male individual and matches the DNA profile from George Michael Zimmerman (JR-2)”

“There is the possibility of an additional contributor to this DNA profile”

–Apparently, there was some slight indication of a DNA mixture.  Not only is there no opinion on whether Martin’s DNA is present, but the testing apparently is not even definitive as to whether the sample is even a mixture.

******As can be seen from many of these conclusions, DNA does not always provide “yes” and “no” type answers!  The fact is that the testing is almost too sensitive for its own good and can detect levels of DNA that are so low that it is impossible to interpret them or put them in a context.  DNA testing is considered ‘esoteric’ testing and is not akin to clinical testing.*******

Stain B 

“gave chemical indications for the presence of blood”

The DNA profile is consistent with originating from a male individual and matches the DNA profile from Trayvon Benjamin Martin (ME-3).” 

–There is no suggestion of a DNA mixture in this stain.

Stain C 

“failed to give chemical indications for the presence of blood” 

–No DNA results were reported.

Stain D 

“gave chemical indications for the presence of blood” 

“The mixed DNA profile obtained from Exhibit ME-8 stain D demonstrated the presence of at least two individuals.  Results are consistent with the presence of at least one male contributor.  DNA profiles for the major and minor contributors could not be determined.”

In this DNA mixture, there is no clear DNA profile present in greater quanitity than other contributors.  In these types of samples, the interpretation is more complicated since single DNA profiles cannot be deduced from the mixture.

George Michael Zimmerman (JR-2) and Trayvon Benjamin Martin (ME-3) are included as possible contributors to the mixed DNA profile obtained from Exhibit ME-8 stain D.”

–DNA consistent with each individual is likely present in similar quantity in the mixture.  Again, there is no comment as to whether there is also DNA present that could not have come from Zimmerman and Martin.

“The combined frequency of occurrence of the mixed DNA profile obtained from Exhibit ME-8 stain D for unrelated individuals in the following populations is approximately: 

1 in 4.0 million Caucasians

1 in 2.1 million African Americans (Blacks)

1 in 6.4 million Southeastern Hispanics” 

–For this sample, I quoted the statistics because they are significantly lower than the statistics presented for the samples which were either single source or for which a clear major profile could be deduced.  These samples had statistics in the trillions, quadrillions, and even quintillions.

While the above statistics are still in the millions, they reflect a significant reduction in the statistical power due to the complexity of mixture interpretation.  These types of mixtures are not as powerful in identifying individuals who may be present.  To put into perspective a statistic of 1 in a million, consider that the population of Florida is roughly 20 million.  It is therefore expected that if you tested everyone in Florida, you would likely find perhaps ~20 individuals who would also be included as possible contributors to the mixture.  While that is a small group, it is in a different league than statistics in the quadrillions (a million billion).  Compare a quadrillion to the World’s population (~7 billion) and I think you see my point!

Stain E 

“gave chemical indications for the presence of blood” 

A complete DNA profile was obtained from Exhibit ME-8 stain E.” 

“This profile is consistent with originating from a male individual and matches the DNA profile from Trayvon Benjamin Martin (ME-3).”

Swabbings from right cuff/lower sleeve 

“failed to give chemical indications for the presence of blood”

“No DNA results foreign to Trayvon Martin (ME-3) were obtained from Exhibit ME-8 right cuff/lower sleeve” 

Swabbings from left cuff/lower sleeve 

“failed to give chemical indications for the presence of blood” 

“The mixed DNA profile obtained from Exhibit ME-8 left cuff/lower sleeve demonstrated the presence of at least two individuals.  Assuming Trayvon Benjamin Martin (ME-3) is a contributor to the mixture, foreign DNA results were obtained.  Due to the limited nature of these results, this data is insufficient for inclusion purposes.” 

“No determination can be made regarding the possible contribution of George Michael Zimmerman (JR-2) to the mixed DNA profile obtained from Exhibit ME-8 left cuff/lower sleeve.” 

–Assumptions are often necessary and appropriate in DNA interpretation, provided they are clearly spelled out.  It is reasonable to assume that Martin’s DNA would be present on a piece of clothing he is wearing.

The conclusion tells us that there is also DNA from someone else, but that the results are apparently sufficiently vague such that it cannot be determined if Zimmerman is included or excluded.

Exhibit ME-12:  Hoodie jacket represented as being from Trayvon Benjamin Martin 

Stain A 

“gave chemical indications for the presence of blood”

“A partial DNA profile was obtained from Exhibit ME-12 stain A.” 

“This partial DNA profile is consistent with originating from a male individual and matches the DNA profile from Trayvon Benjamin Martin (ME-3).” 

–Other language in the conclusion indicates that 11 of the 13 DNA locations tested were used for the statistical calculation, thus the term ‘partial’ DNA profile.

‘Match’ can be a controversial term.  The most important point is that a statistic also be offered to show the significance of the match.  In this case, it is a little surprising that ‘match’ was used to describe a partial profile.

The statistics reported were in the trillions and quadrillions.

Stain B 

“failed to give chemical indications for the presence of blood” 

No DNA results were reported.

Stain C

“gave chemical indications for the presence of blood” 

“No DNA results were obtained from Exhibit ME-12 stain C.” 

Swabbings from cuffs/lower sleeve (both sides) 

“failed to give chemical indications for the presence of blood” 

“No DNA results foreign to Trayvon Benjamin Martin (ME-3) were obtained” 

 

Florida Department of Law Enforcement Report Dated July 26, 2012

Exhibit DMS-22A:  swab from bag of Skittles 

“gave chemical indications for the presence of blood”

A partial DNA profile was obtained from Exhibit DMS-22A.” 

“This partial DNA profile is consistent with originating from a male individual and matches the DNA profile from Trayvon Benjamin Martin..” 

“The frequency of occurrence of the partial DNA profile obtained from Exhibit DMS-22A for unrelated individuals is approximately 1 in 160 trillion” 

“There is the possibility of an additional contributor to the partial DNA profile…..” 

–No comment is offered regarding whether Zimmerman is a possible contributor, since it is not even clear whether there really is a DNA mixture.  Quick note: sometimes data may or may not be from artifacts in the testing process.  If there is only one indication of a mixture, and it could potentially be an artifact caution is warranted in the interpretation of its meaning.

Exhibit DMS-22B:  swab from flashlight 

“failed to give chemical indications for the presence of blood” 

“No DNA results were obtained from Exhibit DMS-22B.”

 

I hope that this blog posting provided a better understanding of the DNA report in this case, but more importantly of how DNA is interpreted in general and how complicated it is.  Without reviewing all of the notes, data, etc. behind the testing I as an expert cannot fully determine the significance of the findings.  Certainly laypersons in the media reading only a DNA report cannot either.

DNA results are complicated, and I think it can be surprising to the public just how much “grey area” there is.  The DNA report in this case raises a lot of questions that will have to be answered by experts on both sides who have full DNA discovery.

  1. James F
    December 9, 2012 at 2:39 pm

    Thank you for this very informative article. However, I noticed you linked to an incomplete version of the report.

    Here is a link to a more complete version which includes more notes. Of particular interest is the statement that the hoodie sweatshirt was very damp and had a strong odor similar to mold and ammonia. This seems like strong evidence that the DNA had likely deteriorated, due to improper storage in a damp environment for weeks before testing.

    http://www.cfnews13.com/content/dam/news/static/cfnews13/documents/2012/09/GZ-FLDE-bio-evidence-2-0919.pdf

    If you are interested and have the time, I would value your opinion and analysis of this more complete version of the evidence.

    • December 9, 2012 at 3:08 pm

      Hi James,

      Thanks for stopping by the blog. This is great information that you are providing. I am definitely interested in evaluating and commenting, but can’t promise a timeline right now!

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