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Triangulation

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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. The term "Genetic Triangulation" was coined by genetic genealogist Bill Hurst in 2004.[1]

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.

The basics of Genetic Genealogy Triangulation.

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

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

Blogs

  • Segmentology A blog by Jim Bartlett focusing largely on segment triangulation for autosomal DNA testing

See also

References

  1. William Hurst. New words for this new field of genetic genealogy. Genealogy-DNA mailing list, 16 December 2004.