Jan 25 2017

Forensic Genetic Genealogy… How does it work?

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A couple of weeks ago, we posted an article on how forensic genetic genealogy helped to find a serial murderer in Phoenix, AZ. This week, we spoke with Colleen Fitzpatrick, an internationally recognized forensic genealogist to learn more about the process of genetic genealogy, and how it can be used to help solve future cases.



Written by: Karen Burkhartzmeyer, Promega Corporation


How distant of a relative can enter Y-DNA into a registry for this technique to indicate a surname?

Since all males in a family share the same Y-chromosome, an unknown assailant can be very distantly related to someone whose Y-DNA is included in a genetic genealogy database.  For example, we have Fitzpatricks in our DNA project that share a common ancestor perhaps 800-1000 years ago. The importance of finding a match in a forensic case is not discovering the genealogy of an unknown or where he fits in that genealogy – the importance is having a possible last name as an investigative lead.


Importance of finding a match in a forensic case is having possible last name as investigative lead. Click To Tweet


How many people would it theoretically take before there could be a registry of a significant percentage of the population?

That’s hard to say.  The sampling of the population is irregular – Caucasian Europeans are better represented in the databases than other populations. Also within the Caucasian European population there are variations in extended family size.  While descendants of early New England families from the 1600s number perhaps in the thousands if not the tens of thousands, there are far fewer descendants of Irish famine immigrants from the early 1850s.  I did a comparison between the top most frequent names appearing in the 1990 US census and the largest genetic genealogy databases.  The names were about the same at the top of both lists, with the exception of Hispanic names, which have much lower frequency among genetic genealogists.


For which type(s) of cases does forensic genealogy work best? For which type(s) is it not as effective?

The value of forensic genealogical analysis is in really tough cases when there are no hits in the CODIS database. A 17-locus Y-STR profile can be generated for comparison to the genetic genealogy databases.  A match can indicate the last name of a suspect, which can be used as a lead in the case.

Law enforcement normally uses CODIS autosomal STR markers for DNA identification, but the CODIS markers do not have counterparts in the genetic genealogy world. There is no means of cross-referencing a CODIS profile with the results of any DNA test available to genetic genealogists.


Value of forensic genealogical analysis is in cases when there are no hits in the CODIS database Click To Tweet


Discuss best practices for implementing this technique.

It must be realized that a match to a last name does not ensure that the unknown has that name.  For instance, there is a chance that the person might have been adopted. Coming up with a last name is just a lead. It is important to discuss this with law enforcement who may not be familiar with genealogical research.


What types of DNA registries do you use? What about Direct to Consumer (DTC) companies such as Ancestry.com or 23andMe.com?

I only use the public Y-DNA databases that are online.  The DTC DNA testing companies have proprietary autosomal DNA, Y-DNA, and mtDNA databases that are available only to their customers.  I have no access to them, nor to the CODIS database used by law enforcement.


You use Y-DNA (Y STR) in your work. Is there any place for mtDNA or autosomal DNA in this technique?

This is a great question because it can clear up misconceptions on what is involved in my work.  There are three major types of testing available through DTC testing companies that cater to the genealogical community:  autosomal DNA, mitochondrial (mtDNA) testing, and Y-DNA testing.

The autosomal SNP tests provided by DTC testing companies such as Ancestry.com and 23andMe to the genealogical community are not currently used for forensic identification.  Autosomal testing for genealogical purposes involves complex analysis of about 700,000 SNPs to estimate genealogical relationships based on the number of autosomal segments shared by individuals, the sizes of those segments, and the number of SNPs that reside on those segments. The companies that offer DTC autosomal SNP testing have proprietary databases available only to their customers and usually require saliva samples. Because forensic identification is based on CODIS autosomal STRs and not on SNPS, it is not possible to cross-reference CODIS results with DTC autosomal SNP testing.

A full Y-STR profile, however, is based on 17 Y-STR loci that are also included in standard genetic genealogy test panels. The matching process is basically a Bingo game with the goal of matching the values of the 17 Y-STR numbers to their DTC counterparts.  The difficulty is not in the matching process itself, it’s the enormous amount of data that must be processed to find a match.  It is hard to estimate how many Y-DNA profiles are online, perhaps 300k to 400k.  There is also the experience necessary to interpret a match to determine if it is valid.  Over the last 15 years, there have been occasional changes in marker calibration by various testing companies that have contributed to these databases.  It is also possible to find matches to more than one surname in cases where there has been an adoption, an illegitimacy, or a name change in a family.

Mitochondrial DNA is analyzed by forensic labs, but it is almost exclusively used for confirming the identity of degraded remains by comparison with known maternally-linked family members.  DTC mtDNA testing databases are not needed for this purpose.  Because of the low mutation rate of mtDNA, a match in a DTC mtDNA database might represent a connection along the direct female line even millennia in the past and would not usually represent an immediate family member of a John or Jane Doe.


Learn more 


Colleen Fitzpatrick, PhD, is the founder of Identifinders International, offering forensic genealogical services for law enforcement cold case work, DNA identification, identity fraud and adoption searches. She is also the author of three books including Forensic Genealogy. Fitzpatrick is currently the forensic genealogist on the Abraham Lincoln DNA project. Learn more about Fitzpatrick’s projects by visiting her website or by following her blog.