FORENSICS: What is a “partial DNA match?” Or, what ARE we talking about here?

This blog follows the event of the Albuquerque police using a “partial DNA match” sufficient to hold a man in jail for 17 months.   The news release from New Mexico is vague about what the police are changing in their ‘procedure’ in response to jailing the wrong man The problems for us is “WTF is a “partial DNA match” and what happened in ABU? The quotation is from a commenter on my last post.

We may never know the last question as this event was only uncovered via a money settlement between the wrongly jailed man and the ABUPD.  Details of which I am sure are sealed. The article below sheds some light.

Being a cynic, I bet the ABUPD only recovered a “partial DNA profile” from the crime scene specimen (they used a degraded DNA sample) and then ran with it to CODIS all the while hoping to coerce the suspect to confess. Someone figured this out after 14 months.

Where is “rapid DNA” processing when we really need it?

Similar cases have occurred using such flawed forensics as “bitemark matching.”

The conundrum of definitions

Semantics  confusion exists when connecting the dots regarding whether, in comparing what we know of the ABU case (“partial DNA match”) to what the accompanying article ( from a District Attorney from another jurisdiction) contains about his “partial DNA match” cases. The article contains case examples.

Preliminary conclusion

The ABUPD case does not fit. Thinking the worst, I suggest that the PD used a crime scene sample that was degraded that resulted in a “partial” profile run against the CODIS DNA database.

The Correct Definition

So, in the literature, a “partial DNA match” means a full DNA profile of a suspect that only “matches” in part to a full DNA profiled crime scene DNA sample.  As the writer below states, that is supposed to lead to a suspect’s  “exclusion.”

There are no numbers in the article to enlighten us as to  how many loci are necessary for a “partial DNA match” to allow a prosecutor to follow the pathway (a “familial” partial DNA match) to ultimately obtaining a full profile of some relation who is the actual perpetrator.

ABU police seem to have messed up this step.

A District Attorney talks about “matching DNA at most, but not all loci.”


When a crime-scene DNA sample matches a profile in the database at most, but not all loci, the individual from the database is excluded. However, it is very likely that a close relative is the source of that DNA rather than a random, unrelated individual. These partial matches can be used by investigators to find the individual who left the DNA at the crime scene, providing potentially valuable information in an ongoing investigation. Although a familial search software was not used to help solve the following cases, they represent examples of investigations where a partial DNA match with a close relative was used to solve crimes.
This webpage is dedicated to the dissemination of information about partial match DNA cases in criminal investigations. If you have case studies in which this method was used or articles involving this topic, please send them to us so they can be included here.

  1. Dennis Rader – for more than 30 years Rader eluded police in Wichita, Kansas, killing ten people and taunting police with the monogram: BTK, for Bind Torture Kill. Using a search warrant, investigators were able to obtain a Pap smear specimen from Rader’s daughter, which had been provided years earlier at a hospital in Kansas. A comparison of her DNA profile from the sample and DNA profiles left at the BTK crime scenes led investigators to conclude that she was the child of the killer. This information, along with other materials from the investigation, allowed police to get an arrest warrant for Rader. When confronted with the information, he confessed to the crimes. News Report re Dennis Rader.PDF
  2. Altemio Sanchez – a rapist/murderer known as the “Bike Path Killer” terrorized Buffalo, New York for three decades, murdering three women and raping over a dozen others. Investigators relied on several key pieces of evidence, including a DNA sample from a relative, to narrow their search. They seized a glass that Sanchez used when he dined at a restaurant and determined that the DNA on the glass matched DNA from the crime scenes of the Bike Path Killer. Sanchez admitted killing the three women and was tied by DNA evidence or similarity of attacks to sixteen other rapes. News Report re Altemio Sanchez.PDF News Report re Bike Path Rapist.PDF
  3. David Bruce Bowen – in 1979 eight year-old Kenneth Conrick was tortured and murdered; the case went unsolved for 30 years. Investigators recently obtained a DNA sample from Bowen’s sister, which showed enough similarities to DNA found on the victim and his clothing at the crime scene to allow investigators to obtain a warrant for Bowen’s DNA. Bowen’s DNA profile matched the profile from semen on cords used to strangle the boy. Bowen pled guilty and admitted that he strangled and stabbed the victim after molesting him. News Report re David Bruce Bowen.PDF

Full article with more cases. 

A very good overview of “familial “matching issues from CBS News from 2007.

One reference from the DA’s story:

The First Successful Use of a Low Stringency Familial Match in a French Criminal Investigation, Emmanuel Pham-Hoai, et al. J Forensic Sci, May 2014, Vol. 59, No. 3 Low_Stringency_familial_Search_JFS_2014.pdf




About csidds

Dr. Michael Bowers is a long time forensic consultant in the US and international court systems.
This entry was posted in Bad Forensic Science, criminal justice, CSI, DNA profiling, Forensic Science and tagged , , , , , . Bookmark the permalink.

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