From ISOGG Wiki
Triangulation is a term derived from surveying to describe a method of determining the Y-STR or mitochondrial DNA ancestral haplotype using two or more known data points. It is also used in autosomal DNA testing to determine the common ancestor by triangulating from known relationships. The term "genetic triangulation" was coined by genetic genealogist Bill Hurst in 2004.
Triangulation has been around a long time. It has been used in maps, navigation and within different sciences. For more information, please see: Triangulation types.
The basics of triangulation for Y-DNA testing
Genetic genealogical triangulation is rather simple. Think of a triangle. /_\
Person A & B match genetically and that forms the base of the triangle. _
Person A has a paper trail (genealogy) that goes back in time. /
Person B has a paper trail that goes back in time. \
The top of the triangle is the MRCA or most recent common ancestor.
For Y-DNA testing: Person A is who you are testing. Some living biological male 2nd, 3rd or better cousin could be Person B. The most recent common shared ancestor is the MRCA.
For other DNA testing: Person A is who you are testing. Another person is Person B. The most common shared ancestor is the MRCA.
If the genetics of Person A & Person B match and the paper trail goes to the MRCA, then this helps prove they are related both genealogically and genetically. This is the goal of genetic genealogy. The genetics help confirm the paper trails (genealogy) back to the MRCA. When this is repeated several times back to a common ancestor, we then can recreate the DNA markers or genetic fingerprint of that ancestor. All without digging them up!
If there is a break in any point of the triangle, it should be noted appropriately.
- If Person A & B match genetically but either paper trail (genealogy) does not go back to the MRCA, then they match genetically but not genealogically.
- If Person A & B do not match genetically, but match with the paper trails, then they match genealogically, but not genetically. In this case the genealogy may be wrong or there is a formal or informal adoption of DNA into the genealogical line. The later is called a non-paternal event.
When comparing any DNA test using triangulations, one should always cite the common test. For example, When comparing say a 37 marker Y-DNA test with 111 marker Y-DNA test, you should always cite the lower value. Using the example given, a proper statement of genetic triangulation would indicate that Person A & Person B matched genetically and genealogically at 37 Y-DNA markers.
Further reading on Y-DNA triangulation
- One example of a reconstructed Y-DNA haplotype that used Genetic Triangulation. See: William Carpenter of Providence, RI.
- Triangulation An essay from the Taylor Family Genes Project
- Charles Kerchner's definition of triangulation
- Triangulation in genetic genealogy by Emily Aulicino, Genealem blog, 25 March 2008
- Triangulation for Y-DNA by by Roberta Estes, DNAeXplained, 18 June 2013.
Triangulation with autosomal DNA testing
In autosomal DNA testing the term triangulation is most commonly used to describe the process of reviewing the pedigree charts of clusters of shared matches/in common with matches in order to identify a common ancestor or ancestral couple. This process is sometimes also known as tree triangulation.
Triangulation is also used to describe the process of identifying shared segments of DNA and trying to identify a shared ancestor or ancestral couple from whom the segment has been inherited. This process is also sometimes known as segment triangulation. Segment triangulation requires access to segment data which is not available at AncestryDNA. Segment triangulation is best used in conjunction with chromosome mapping.
There are a number of third-party tools which can be used to aid the process of segment triangulation such as those available from GedMatch and DNAGEDCOM (eg, Don Worth's Autosomal DNA Segment Analyser).
Further reading on segment triangulation
- Tips for triangulating your DNA matches by Diahan Southard, Family Tree Magazine (US).
- Triangulation is the icing not the cake by Shelley Crawford, Twigs of Yore, 3 March 2018.
- Triangulating autosomal DNA National Genealogical Society NGS Magazine 42 (October-December 2016): 39-42.
- DNA update by Barbara Griffiths. Not just the Parrys - a genealogy blog for all my ancestors, 25 November 2016. Includes a discussion of triangulation from a UK perspective.
- A triangulation intervention by Blaine Bettinger, The Genetic Genealogist, 19 June 2016.
- TG review: 12 kits at FTDNA by Barton Lewis, Barton Lewis's Genealogy Pages, 29 May 2016. An analysis of triangulated groups in 12 kits at Family Tree DNA.
- A triangulation feature on the new 23andMe by Kitty Cooper, Kitty Cooper's Blog, 26 May 2016.
- Autosomal DNA triangulation Part 1: the basics by Debbie Kennett, Cruwys News blog, 28 January 2016.
- Autosomal DNA triangulation Part 2: the phenomenon of triangulated segments by Debbie Kennett, Cruwys News blog, 28 January, 2016.
- An analysis of fourth cousins and other near distant relationships by Jim Owston, Owston/Ouston One-Name Study blog, 10 August 2015. Includes an exercise in triangulation.
- How to triangulate by Jim Bartlett, Segmentology blog, 11 May 2015.
- Genealogy and autosomal DNA matches: common errors in proving an ancestor and the allure of easy gateway ancestors Genealogy and Genomics blog, 19 April 2015. (Retrieved from the Wayback Machine, 3 January 2021)
- The trouble with triangulation: preliminary notes by Ann Turner, 4 April 2015.
- Triangulation: proving a common ancestor by Kitty Cooper, Kitty Cooper's blog, 26 February 2015.
- Technique for segment triangulation when GedMatch tool not available by Sue Griffith, Genealogy Junkie, 28 August 2014.
- Triangulation for autosomal DNA by Roberta Estes, DNAeXplained, 21 June 2013.
- Triangulate to find more meaningful matches using both Family Tree DNA and 23andMe by Randy Majors, Randymajors.com, 15 May 2012
- Segmentology A blog by Jim Bartlett focusing largely on segment triangulation for autosomal DNA testing
- William Hurst. New words for this new field of genetic genealogy. Genealogy-DNA mailing list, 16 December 2004.