AncestryDNA and MyHeritage have recently released new tools for showing how you might be related to your DNA cousins. Both companies look at your tree and the tree of your DNA cousin. If those trees do not connect, the company tentatively connects them using in addition other public trees in their databases.
Cousinships found in tentatively-connectly trees of course need to be verified. However, although you may seem to descend in the way suggested by the tentatively-connectly trees it is possible that you could be also descended from other ancestors not presently shown on the family trees. Confidence that the DNA match is from a particular couple increases with tree completeness. This confidence increases for both you and your match if your are able to establish that there are likely no other shared ancestors. Increased confidence that you are indeed related to the suggested DNA cousins comes from sharing common ancestors and simultaneously sharing DNA other cousins who also descend from the same common ancestors
AncestryDNA’s new Thrulines tool has given me many more Common Ancestor Hints to look at. I decided to look at DNA cousins who are also descended from my fifth great grandparents Dominique Pierre La Mothe (1731 – 1807) and his wife Susannah La Mothe nee Corrin (1741 – 1803). ThruLines suggests that my father may be related to 8 DNA matches through Dominique Pierre Lamothe.
One of these matches is me; another match is one of my second cousins, my father’s first cousin once removed. These relationships are well documented. We have met my father’s third cousin, R, and I also have confidence in the documentation of that match. However, we have not met H, my father’s 3rd cousin once removed. The relationship corresponds with my family history researches, and the shared 21 centimorgans of DNA fits within the range of a 3rd cousin once removed relationship as predicted by the shared centimorgan tool at DNAPainter.com.
Four matches descend from other children of Dominique and Susannah La Mothe.
These matches were all new to me. I had not previously corresponded with these cousins, nor was I aware that our trees had common ancestors. The shared DNA is small, but is greater than the 7 centimorgan small-match-limit usually suggested for genealogical significance. The number of shared centimorgans corresponds with the hypothetical relationships.
I decided to review cousin C who is predicted to descend from Frederick John Dominique Lamothe (1805 – 1864). I had not previously documented his descendants. The family tree attached to C’s DNA has only 27 people: she lists paternal grandparents and her mother, who died in 2015; there are no maternal grandparents listed. Thrulines incorrectly shows C’s grandfather’s information as common from her tree, but that does not correspond with the only public member tree attached to her profile, so perhaps C has a private but searchable tree on Ancestry.com.
I traced 13 children of Frederick John Lamothe, of whom five were daughters. His youngest daughter Ann Jane Lamothe (1857 – 1929) married William Galloway (1854 – 1909) on 5 August 1879 at Lezayre, Isle of Man. They had ten children. The second oldest was William Edward Galloway (1884 – 1967). That he was the father of Jean (1927 – 2015) is documented on a 1951 US Border-crossing document from Canada to the US. Her husband is also named in the document, further confirming the family relationship to C. I am satisfied with the genealogical links between C and Dominique Pierre and Susannah Lamothe based on baptism, marriage, death, and census records as well as the border-crossing record.
My father has eight matches at MyHeritage where MyHeritage has build speculative trees that may explain how Richard Rafe Champion de Crespigny and some of his DNA Matches are related. Of these I had already determined the connection for six and been in contact with the six cousins. The other two matches are his cousins, brother JJ and sister MJ, whose DNA kits are administered by Jo M, the daughter of JJ. My father shares 31.3 centimorgans of DNA with MJ and 8.3 centimorgans with JJ. The shared DNA figures are within the range appropriate for 5th cousins.
My Heritage demonstrates how the two trees combine and gives a level of confidence about the match, in this case 82%. While I did not have these cousins on my tree previously I have now added these descendants to my tree on the basis of birth, death, marriage, and census records.
MyHeritage provides a chromosome browser and lists segment details. I have painted these matches on to my father’s DNAPainter profile. The overlaps of the segments all correspond with forebears who descend from Dominique Pierre Lamothe and Susannah Corrin. To date 25% of my father’s DNA has been attributed to named forebears.
In his talk Essential Considerations for DNA Evidence, presented at RootsTech 2019, Blaine Bettinger, an American genetic genealogist claimed that “Without a report of tree completedness, it is nearly impossible to evaluate the use of DNA in a genealogical conclusion, even if it is soundly supported by documentary evidence!”
I last looked at tree completeness in May 2018 when I could name 45 of the possible 64 5th great grandparents I had. I did not split my result between my father’s family history and my mother’s but when I reviewed the statistics, it appeared that 44 of my possible 5th great grandparents that I know of are on my father’s side. That is, I know the names of 68% of my father’s fourth great grandparents, the generation that includes Dominique and Susannah LaMothe. I have improved my knowledge slightly. I now know 70% of my father’s 3rd great grandparents and 42% of my father’s 4th great grandparents, i.e. those forebears he would share as most common recent ancestors with fifth cousins. The overall tree completeness score at the 5th cousin level for my father is that we know 106 of a potential 127 individuals or 83%: 17% of the tree is unknown.
In the case of the Thrulines match with C, her tree has only 27 people and could not be regarded as complete.
In the case of the MyHeritage tree maintained by Jo M and associated with the matches of MJ and JJ, the tree has only 84 people and is also incomplete. Jo M has trees on Ancestry.com but they show that Jo M has shown only 41 of the people associated with the trees of JJ and MJ up to the 4th great grandparent level, or 32%; thus her tree could be said to be 68% incomplete.
AncestryDNA’s Thrulines tool and MyHeritage’s Theory of Family Relativity tool are similar. Both tools have come up with matches at the fifth cousin level that seem plausible. In verifying the lines of descent and contacting the matches, I have discovered a little more about the descendants of my forebears. One cousin, Jo M, has shared pictures she took of our forebears’ graves on the Isle of Man. She has also traced the La Mothe family line further back than I have.