Is it the math or the tangled relationships that make using genetic DNA so hard to grasp? Sometimes it seems to be the proliferation of shiny new tools used to manipulate DNA results whose terms and possibilities are quickly adopted by those in the know which seem to leave me in the dust. Then again, maybe it's just my lack of practice.
How I envy those with compliant relatives willing to share their spit or cell scrapings. But it's time to stop dreaming about "what if" and working with the information that I have. To further my education, I signed up for a week long course about genetic DNA at the Society of Genealogists which just happened to be on while I was in London. I still haven't unpacked a lot of the information from the sessions but I did buy a new book that was recommended.
The book is Advanced Genetic Genealogy: Techniques and Case Studies edited by Debbie Parker Wayne. It covers many DNA related topics. The first one I looked at was Kimberly Powell's article on "The Challenge of Endogamy and Pedigree Collapse". Turns out that I have pedigree collapse in my line with the odd number of matching CMs for a distant cousin match. This line reaches back to the early 1600s so nothing is written in stone but the information I was given shows multiple tangled connections.
It is the tangled intermarriages before the common ancestors shared by me and my target match that, I believe, explain the odd CM number that my distant cousin and I share. Working down through the generations since our shared ancestral couple, my match and I are 4th cousins once removed which doesn't explain the 54CM DNA match. According to the "Average autosomal DNA shared by pairs of relatives" chart on the ISOGG website, average shared CMs for 4th cousins once removed are 6.64. Even 4th cousins have only 13.28 average shared CMs. But what about multiple intermarriage connections which, in this case, are thought to precede the common match? In that case, my match and I would be 4th cousins once removed more than once.
According to the article in Advanced Genetic Genealogy, the probable shared CMs can be calculated by figuring out each shared relationship and adding the probable CMs together. Well, there was intermarriage in the background of the ancestral couple which my match and I share. Quite a bit of intermarriage.
As the family trees show, Lydia, the granddaughter of John Tripp and his great grandson, Jonathan Tripp married. They were 1st cousins once removed.
But they were more than that as their mother's were sisters which made them 1st cousins on the maternal side as well. This meant that my match and I were related on 4 lines which, I believe, makes us 4th cousins once removed 4 times which would give us a probable amount of shared CMs of 26.56 (4 x 6.64) according to the probable shared CMs on the ISOGG chart. There is always a degree of variation in the shared CMs which may account for the actual CMs being 54 or perhaps there is further pedigree collapse in this line or even endogamy as this line is to Colonial American ancestors. On a more personal level, the inter-relatedness of Lydia and Jonathan Tripp does given me pause.