My father has a second cousin once removed, ‘GH’. I am GH’s 2nd cousin twice removed.

GH’s daughter administers his DNA kits. She asked me an interesting question. On the DNA evidence, is this the same person:

  • Ancestry – AnneYoungAU on Ancestry (shares 33 cM with my father)
  • Family Tree DNA – Christine Anne Young (shares 93 cM with my father)
  • GEDmatch – Anne Young A947648 (shares 63.5 cM with my father)
  • My Heritage – Anne Young (shares 56.3 cM with my father)

If they are the same person, how can the results be so different?

Also, on the DNA evidence, are these the same person:

  • Ancestry – RRC001 (shares 110 cM with my father)
  • Family Tree DNA – R. Rafe Champion De Crespigny (shares 134 cM with my father)
  • GEDmatch – RD A587626 (shares 113 cM with my father)
  • My Heritage – Richard Rafe Champion De Crespigny (shares 98.3 cM with my father)

DNA aside, of course, we know that they are the same two people. The first is me, the second is my father.

The reason that the amount of shared DNA seems to vary so widely has to do with different assumptions and techniques used to calculate genealogically significant DNA. As a species we share DNA with other forms of life, but much of this is irrelevant genealogically. Discarding the parts we share as living beings and concentrating on what we share as family relatives introduces different emphases. The result is apparent differences between DNA analysis by different companies.

AncestryDNA explains it in these terms:

“If you choose to upload your AncestryDNA raw DNA results to another website, they will look different for a number of reasons. Other companies do not use the same algorithms, database or methods to translate the data. Only AncestryDNA has access to the unique information available on Ancestry, including the family trees and records to help power the accuracy of the results. In addition, the proprietary algorithms that we use to calculate results are based on documented family trees and a one-of-a-kind, comprehensive database of DNA samples from around the world.”
Curious about the detail, I used DNAPainter, an online chromosome mapping tool, to investigate a little further.

I was interested to see the variation in what segments my father and I shared with GH so I painted the matches from MyHeritage, FamilyTreeDNA and GedMatch at DNAPainter using the default parameter excluding segments under 7 centimorgans and also experimenting with including all segments greater than 1 centimorgan (the lowest threshold allowed).

Reviewing the data for segments of 7 centimorgans or more I found that MyHeritage did not include matching segments on chromosome 1. The segment matched with GH corresponds to a known pile up area and is indicated with grey shading on the DNAPainter diagram. It probably indicates shared DNA without genealogical significance.

dna painter profile for gh 7 cm compare

DNA profile comparing matches for GH with Rafe and Anne: 7 centimorgan threshold

Reviewing the data for all segments greater than 1 centimorgan I found that Family Tree DNA and GedMatch included these segments whereas MyHeritage did not. I shared some small segments with GH but my father did not indicating that I did not inherit these segments from the most common recent ancestor GH, my father and I share. In general 7 centimorgans is considered necessary before a segment is possibly inherited from a common ancestor.

dna painter profile for gh 1 cm compare (1)

DNA profile comparing matches for GH with Rafe and Anne: 1 centimorgan threshold

The MyHeritage estimates of shared DNA make sense in view of the 7 centimorgan threshold and by not including the shared DNA at the pileup region on chromosome 1. Unfortunately because the detail of the match at AncestryDNA is not revealed by that company I cannot comment on what data they chose to include or exclude for their match.

All this is a reminder that DNA matching, though a technique of great precision, makes certain assumptions, and operates within recognised limits. Its apparent accuracy will occasionally seem to vary. Be careful how you interpret DNA results. You are indeed related to the fly  you just swatted- you murdered a distant cousin – and you are related to yourself, since your DNA, though analysed differently by different companies is all yours. Somewhere between you and blowflies are your cousins and other family members, not all of whom you’d want to claim for immediate relatives.

Further reading

  • “International Society of Genetic Genealogy Wiki ISOGG Wiki.” Identical by Descent, International Society of Genetic Genealogy, 23 Nov. 2018, isogg.org/wiki/Identical_by_descent.