Familial DNA Searching

It's all in the family

• Suzanna Ryan
• 2008 Oct 21

Imagine this scenario--you are involved in the investigation of a high-profile serial rape case and you are lucky enough to have a DNA profile taken from the victims' bodies. The DNA profile links all the rapes, but despite your best efforts, you have no suspect. Worse yet, when the profile is searched in both the state and national DNA databases there is no matching profile. Now imagine there was one more step you could take to help you solve the case, but the state you live in, as well as the federal government, won't allow it. That hypothetical additional step  does  exist. It is often termed Familial DNA Searching and it has been in use in the United Kingdom for the past five years. Most states in the U.S., however, are hesitant to allow this type of search even though, according to a recent  Washington Post article, some experts believe that Familial DNA Searching could increase the number of suspects identified through their DNA by up to 40%. 

How does Familial DNA Searching work?
Familial DNA Searching works because closely related individuals share more of their DNA than those of unrelated individuals. This is the same principle that is employed in paternity testing and missing persons identification. The searching technique employs the use of DNA databases and is based upon "partial matches" between samples. In the scenario above, the DNA sample from the crime scene did not match any of the over 6 million convicted offender DNA profiles in the database. If a lower stringency search is used, CODIS (the Combined DNA Index System) will allow for a near-perfect match to occur and not just a perfect match. These near-perfect matches may very well be family members (i.e. siblings or parents) of the rape suspect since family members like fathers and sons share 50% of their DNA. The information gleaned from a low-stringency CODIS search can then theoretically be used as investigative leads to narrow the search in identifying the perpetrator.

Privacy concerns
Some of the hesitation to use this type of searching in the US is due to the objections of privacy advocates, such as the American Civil Liberties Union, who argue that this type of search is tantamount to "genetic surveillance". They also insist that it could make individuals whose DNA profiles are in the database unwilling informants about their family members. The ACLU is especially concerned about the use of this type of database search in those states that collect DNA from persons  arrested  for, but not necessarily convicted of, specific crimes. 

The identification of the BTK Killer
One of the most famous examples of this type of familial or kinship comparison is with the identification of the BTK killer in February 2005. Although this identification did not involve a database search, it did use the DNA from Dennis Rader's daughter to help identify him as the BTK Killer. This comparison was made without the daughter's knowledge. Her DNA was typed from a Pap smear specimen she had given at a university medical clinic while a student. The daughter's DNA profile was compared to DNA found at various crime scenes linked to the BTK Killer and due to the large percentage of shared DNA between the two samples, an arrest warrant was issued for Dennis Rader. 

Government views on Familial DNA Searching
The FBI has indicated discomfort with allowing familial DNA searching of the national DNA database. According to the head of the FBI's national DNA database, Thomas Callaghan, (who was quoted in a recent  Washington Post article), "The FBI would be more comfortable with congressional authorization to conduct familial searches." In addition, most states do not expressly allow familial searching. Recently, though, California has become the first state to create a policy to allow familial searching. In April of 2008, the  LA Times reported "California will adopt the most aggressive approach in the nation... to identify elusive criminals through their relatives." According to state Attorney General Jerry Brown, the technique will only be used when all other leads have been exhausted. Colorado and Massachusetts are also on the forefront of drafting policies to allow police agencies in their states to utilize the searching technique. In fact, the Denver District Attorney has an entire webpage, "dedicated to the dissemination of information about familial DNA searches in criminal investigations".

An imperfect system
While many forensic experts believe that familial searching would be a beneficial crime fighting tool, one problem in performing this type of searching is that CODIS was not designed for this type of searching. It was designed to indicate when two samples match each other perfectly--at all 13 locations on the DNA molecule that are compared. In a discussion on Charles Brenner's Forensic Mathematics website regarding familial DNA searching, Brenner opines that the number of potential false matches utilizing a "low-stringency" search of the CODIS database is approximately 40/50,000. In a database consisting of only 50,000 DNA profiles, you will find 40 false familial matches in addition to the one possible true familial match. Brenner instead advocates a "hybrid searching approach," consisting of the use of likelihood ratios to help take into account samples that share rare alleles--these samples are much more likely to show a partial match due to a true family relationship than simply by chance. Brenner also recommends Y-STR typing of those samples that do indicate a potential familial relationship to help weed out the false matches (since Y-STRs are inherited patrilineally). This proposed method is similar in nature to the system used by the Forensic Science Service of the UK. The FSS has had success with familial searching and has solved several high -profile cold cases by using this method over the past five years.

With California in the lead, it seems to be only a matter of time before more and more states begin drafting policies to allow familial DNA searching. There is no doubt that there will be a large amount of resistance to this technique from privacy advocates and potentially from the courts. However, if well thought-out policies that factor in privacy concerns are put into place, this technique could be a real boon to detectives who have hit dead ends in their investigations. 

• More articles by Suzanna Ryan
• Familial DNA Database Searches (Denver District Attoney's Office)
• State offers police extra DNA tool (at LATimes.com)
• From DNA of Family, a Tool to Make Arrests (at WashingtonPost.com)
• Forensic Mathematics

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• Suzanna Ryan

  Suzanna Ryan, MS is a professional forensic DNA analyst and consultant with ten years of experience. She also teaches graduate courses in forensic science near her home in San Diego. >>View Author Profile