DNA Diary: An update ~ and guess who our new cousin is…

According to FT DNA, the company who tested our autosomal DNA, their Family Finder tests are best placed to help ‘break through brick walls’.1  Unfortunately, it is not as easy as it sounds. Our results are out since March, my mother has 475 matches, but we are no closer to identifying a single new ancestor.  These 475 matches are deemed to be related because they share sufficient linked segments of DNA. The more segments two people share and the longer each segment is, the greater the chance that they are related.  The common ancestor with most of these matches lived so long ago that surviving documentation will never ‘prove’ the relationship, but, there are a number of third cousins listed, which at first glance look promising.

It quickly transpired that autosomal DNA tests are really only useful where both matching ‘cousins’ have a fairly complete family tree with only the odd gap to be filled. Autosomal DNA is only useful going back about 4 or 5 generations, as the amount of DNA inherited from individual more distant ancestors is just too small. Yet, even in this recent time-scale, most of our ‘cousins’ have as many, if not more, gaps in their tree than we do, at least on their Irish lines, so it became increasingly obvious that their trees were unlikely to help fill our gaps.

Then, my known third cousin, Phyllis agreed to upload her DNA results to GEDmatch, a site that freely hosts DNA data from various testing companies. We could then compare her DNA to my mother’s and to my maternal uncle’s. GEDmatch confirmed that they shared a ‘Most Recent Common Ancestor’ within the last 3 generations (3.7 and 3.5 to be exact), I guess falling within the parameters of them being second cousins once removed.

Documentary evidence has proven that Mam, Uncle C and Phyllis are all directly descended from John Wynne and Bridget Hynes. So, by comparing their DNA, we could ‘label’ the shared segments, specifically the longer ones, as being probably inherited from John and Bridget. Thus, if we shared the same long segments with someone else, we could deduce, at least initially, that the relationship was probably via John and/or Bridget too and then we might be better able to identify our common ancestor.   

Then I noticed that Phyllis and my mother matched on their X chromosome. Given the specific inheritance path of X, they should not have, but, they shared a number of segments, the largest being 8.2 cMs across 902 SNPs. Everyone inherits an X chromosome from their mother. Females also inherit an X chromosome from their father. Males get a Y chromosome from their father instead (XX = female; XY = male). So, when my mother received her X chromosome from her father, it was a replica of her paternal grandmother’s, i.e. Teresa Carroll’s, and contained no X DNA from her paternal grandfather, Patrick Wynne, hence none from John and Bridget.

It seems unlikely that this match is a mere coincidence, although, their donating ancestor may well have lived a very long time ago. It means that they are double cousins and any labeling of the shared segments on the other chromosomes is less reliable, as a result. We need another known Carroll third cousin to take the DNA test to isolate any shared Carroll DNA. Actually, we need a lot more of Mam's second and third cousins to take the test, full-stop.

Here's the surprise! GEDmatch also suggests that my mother and Aunty C (Uncle C’s wife and Mam's sister-in-law) are related within the last 4.5 generations, so third or maybe fourth cousins. Their longest shared segment is 12.6 cM. FT DNA also records the match, but puts it in the ‘5th cousin to remote’ bracket. Both companies show that Uncle C and Aunty C have very little DNA in common and are not a match, so any match with Mam could be fairly ancient. Given the population size of Ireland, most of us are probably related, if you go back far enough, which is probably not helpful. Anyway, just in case it is true, welcome to the family, again, Aunty C! :) 

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1 FamilyTreeDNA, accessed 1 May 2014.

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