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New study of DNA interpretation

Oct 5, 2017

We hope that the science is better understood by the researchers at NIST than whoever wrote what appears to be an official document describing new research in DNA interpretation of mixtures.

“The study will focus on DNA mixtures involving three or more people, and on very small quantities of DNA also known as touch DNA.”

We hope that the science is better understood by the researchers at NIST than whoever wrote what appears to be an official document describing new research in DNA interpretation of mixtures.

“The study will focus on DNA mixtures involving three or more people, and on very small quantities of DNA also known as touch DNA.”

First, the term ‘touch DNA’ is a misnomer and misleading in court as it is known that a person’s DNA can be found on an item without the person having touched it.

Second, the statement later says;

“Rigorous scientific studies have shown that when the evidence contains DNA from only one or two people, DNA profiles are extremely reliable.”

Really?  Maybe when one of the contributors is known, but when there are no known contributors there is no real difference between the difficulties of establishing the number of potential contributors or identifying their profiles.  Two-person mixtures generate billions of potential contributors; the issue is, as with all mixtures, who are the actual contributors? 

What is meant by ‘DNA profiles’?  The graph that contains information about the DNA components, or the statistic that informs as to whether it is probable that a contributing profile could match a specific person?   DNA profiling consists of interpreting the result – i.e. which components are present – and then attempting to identify the contributors on the basis of those.  The writer seems to be confusing or conflating those two separate, but crucial, tasks.

“But when the evidence includes a mixture of DNA from three or more people, it can be difficult to tease apart the different profiles, or in some cases, to even determine how many profiles are present.”

It is NEVER possible to determine with certainty how many contributors there are to a DNA mixture, only the minimum number.

“The goal of the study is to measure the reliability of DNA profiling methods when used with different types of DNA evidence such as two-person mixtures versus four-person mixtures, and with different quantities of touch DNA.”

Didn’t they just say the goal was to look at mixtures of more than three?

The scientific literature contains many hundreds of papers researching these very topics.  It seems unlikely that one further study, no matter how extensive, will either settle the current extensive debate on the interpretation of complex DNA mixtures obtained from crime scenes or be sufficient to, “decide which methods pass muster” – but we look forward to seeing the published results.  It is, at least, arguable if anyone has the resources to construct experiments that explore the range of circumstances found in real casework.

 

 

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