Locus 1 17, 18
Locus 2 7, 7
Locus 3 29, 30.3
Locus 4 18, 18
Locus 5 11, 12
Locus 6 10, 13
Locus 7 9, 10
Locus 8 9, 9.3
Locus 9 10, 10
Locus 10 11, 14
Locus 11 12, 12
Locus 12 8, 11
Locus 13 24, 27
Gender Locus X, Y
Rest assured that the data shown above was NOT derived from any actual person. This person—if real—would be a male (as indicated by the ‘XY’), and the names of the loci are NOT Locus 1, Locus 2, etc. The actual human loci have tricky designations like vWA, TPOX, and DS1358, etc. If a DNA profile, resembling the example show above, did indeed come from a particular individual, this male would almost certainly be the only person on our planet (other than an identical twin) with this precise variety of genetic markers. Doubts concerning the rarity of forensic DNA profiles have been debated for a few years now. For more info on this, look at this L.A. Times article covering THE FBI, ARIZONA DNA DATABASE SEARCHES, AND THE BIRTHDAY PARADOX.
It is important to note that—with rare exceptions—each human has two DNA markers at each locus. These markers are often different from each other (example from above: the 17 and 18 at Locus 1), but sometimes we receive—from our mom and our dad—two copies of the same DNA marker (example: the 7 and 7 at Locus 2).
Imagine that you are a forensic biologist, working in a police crime laboratory. If you want to take it a step further and pretend that this is your favorite TV drama, CSI: Miami, be my guest. Perhaps the murder weapon—a hunting knife—was left at a downtown Miami crime scene. The DNA swab from blood on the knife blade clearly established that the victim was stabbed with this knife.
When you collect a second swab from the knife handle, you are hoping to detect a single-source DNA profile, presumably, left by the murderer—who recently handled the knife. Hopefully, that DNA profile, would appear somewhat similar to the pattern of numbers in my fictional example—shown above. To your dismay, ….there are WAY TOO MANY DNA markers on the knife. At a few loci, you see two genetic markers, but at other loci, you observe, three, four, five, ….and in one case, a locus with SIX genetic markers.
Welcome—rookie forensic biologists—to the world of DNA mixtures. When you compare your ‘inventory’ of DNA markers from the knife handle, you see little or no resemblance to the DNA profile from the victim’s blood that you have already detected on the knife blade. That was a job well done, as you intentionally AVOIDED collecting any blood residue when you prepared the swab sample from the knife handle. At this stage, you can ONLY conclude that multiple individuals have—at some point in time—deposited DNA on this knife handle. You cannot say—with certainty—that this is ‘touch DNA'—since you have conducted NO TESTS to identify the type of cells on the knife handle.
You also cannot say—precisely—how many individuals have left some DNA on the knife handle. However, the presence of SIX DNA markers at one locus, indicates that it was AT LEAST three individuals. If SEVEN DNA markers had been detected on one or more loci, you would have to assume AT LEAST four contributors of DNA.
Now comes the challenging part. Your esteemed associates with the local police department have apprehended a promising suspect. Since this suspect has been convicted of a variety of past crimes, the state already has his DNA profile on record. Eagerly, you access this information to see if this man’s DNA profile shows similarities to the abundance of DNA markers detected on your knife handle.
The excitement builds as you begin to see distinct similarities. Eureka! As you continue to scan the data, you become a bit dejected to see that the apprehended suspect has a 16 DNA marker at Locus 4. But NO 16 marker was detected at Locus 4 on your knife handle! You begin to wonder, "How should I interpret this?"
Do not despair, ….there is such a thing in forensic biology as ALLELIC DROPOUT. When a PARTIAL DNA PROFILE is observed, some DNA markers might be detected, ….whereas others might not show up among the data. This could be due to the fact that the contributor simply did not leave behind enough DNA to get a full-profile. As you think this through, you realize that this ‘allelic dropout’ effect could certainly happen within the context of a DNA mixture. ONE lousy DNA marker somehow evaded detection, ….so what?
As you continue your evaluation, you find that the suspect has a total of 22 different DNA markers. All but ONE of those markers (that nasty little 16 ‘dropout’ at Locus 4) are present within the mixture of FORTY-SIX DNA markers on the knife handle. Who are those other two individuals-contributing DNA to your knife handle? Who cares? These data look pretty good after all!
As you settle in to write your DNA report, your crime lab has provided you with clear guidelines as to how the results of DNA comparisons must be summarized. Upon comparing a known DNA profile (from the suspect) to a DNA mixture—such as the 46 DNA markers detected on the knife handle—the majority of crime labs provide the analyst with a few distinct reporting options: 1) The analyst can report that there is a perfect, single-source match between the known DNA from the suspect and the DNA on the knife handle; 2) The analyst can report that the suspect is EXCLUDED as a potential contributor of DNA to the mixture of markers on the knife handle; 3) The analyst can report that the suspect cannot be excluded as a potential contributor to the DNA mixture on the knife handle; 4) The analyst can report that the DNA data are insufficient to reach any reliable scientific conclusions—without the benefit of more testing and additional data.
Option 1 and Option 2 CLEARLY do not fit the data. There is no single-source match. There are at least 24 unaccounted for DNA markers on the knife handle and one of the suspect's DNA markers is entirely absent. Excluding the suspect as a potential contributor is equally incorrect. Similarities between his DNA and the observed mixture indicates that his genetic material may be present as component of the mixed DNA. This leaves ONLY the last two options. Arguments over Option 3 and Option 4 continually spawn countless, heated courtroom battles.
Trust that I have reviewed many DNA mixture cases—involving much more than 46 total observed genetic markers and more than just one ALLEGED allelic dropout. Sometimes the crime lab analyst refuses to admit that the observed data are all but worthless. In some of those cases, there were multiple suspects, multiple victims, and possible contributing bystanders, ….including individuals who are known to be GENETICALLY RELATED.
The unfortunate bottom line is as follows: There are no clearly defined and accepted, universally-enforced guidelines for the analysis of DNA mixtures in criminal cases. Furthermore, there are no precise guidelines for how any DNA mixture should documented in lab reports and explained to jurors. Some may argue that guidelines ARE being proposed, documented, and debated. However, I have personally witnessed little more than a multitude of disjointed, loosely defined sets of instructions—often quite vague—from city to city, county to county, and state to state.
For this reason, Spence Forensic Resources has outlined a policy on TRIAL PREPARATION, with a focus on scrutinizing/interpreting DNA mixtures observed by the various reporting laboratories. The steps for these interpretations are as follows:
Step 1: Analysis of electropherogram data to ensure that the profile in question is, indeed, a mixture. When a single source DNA profile from an evidence item matches the profile of a known reference standard, with the exception of ONE additional allele, it is my opinion that no statistical conclusions should be drawn from that single allele. This is especially true when the single allelic peak height is relatively low, or can potentially be attributed to an artifact of the PCR process or capillary electrophoresis.
Step 2: In the event that a genuine mixture is being called, SFR will evaluate the mixture for the possible presence of a major profile, plus one or more minor profiles. The alternative to this would be the apparent presence of NO major profile, ONLY minor, indistinguishable sources of DNA. This evaluation is dependent upon a detailed assessment of the peak height landscape of the entire mixture profile electropherogram-with consideration of the potential contributing reference standard profiles. A final determination will be at the discretion of the analyst, based on years of experience in evaluating hundreds of DNA profiles.
Step 3: If three alleles are present at two or more loci, the mixture includes at least two individuals. If five alleles are present at one locus or more loci, the mixture includes at least three individuals. If seven alleles are present at one locus or more loci, the mixture includes at least four individuals, and so on.
Step 4: On a case by case basis, SFR will evaluate of the capacity of each mixture to be interpreted through sound, reasonable, scientific methods. A variety of factors can contribute to the conclusion that a DNA mixture is simply unsuitable for a reliable interpretation. These factors include, but are not limited to the following: 1) The number of contributors to the mixture-the greater the number of total alleles observed, the less potential there is for a reliable statistical conclusion. 2) When indistinguishable minor contributors are relatively equal donors of DNA, little can be gained from scrutinizing allelic imbalances. 3) When alleles overlap, such as might be expected from contributors who are genetically related or from the same ethnic population pool. 4) When one or more unknown individuals have contributed to the mixture profile, i.e. they have donated DNA alleles that cannot be correlated with the available known reference standards. 5) Allelic dropout has to be assumed in order to make non-exclusion calls.
Step 5: In the event that a genuine mixture is called, the degree of allelic dropout may be cautiously evaluated for the potential exclusion of each suspected contributor. When a key suspect or victim reference standard profile is to be compared to a DNA mixture, the absence of ONE allele from the mixture could be evidence for potential exclusion. An example of this would be that Suspect A has a 9, 11 DNA profile at a given locus. In the mixture profile, a 9 allele-with a substantial peak height-is observed that cannot be attributed to any other suspected contributors to the mixture. Meanwhile, there is a clear absence of an 11 allele in the mixture. Observation of additional dropout events corresponding to Suspect A would further strengthen the cause for exclusion. By assessing the peak height landscape of the DNA mixture electropherogram and the number of apparent allelic dropout events, SFR will utilize years of experience-evaluating hundreds of DNA mixture profiles to arrive at decisions on exclusion or non-exclusion.
Step 6: When a mixture is judged to be suitable for interpretation and there is insufficient evidence for exclusion, the probability of exclusion will be calculated for the mixture. If there is evidence of allelic dropout at a particular locus, that locus will not be used in the probability of exclusion calculation for the mixture.
The next article is entitiled:
The Casey Anthony Media Circus
Michael J. Spence, Ph.D.
January 11, 2012