The Misunderstood Short Segment Match

When I started this blog late in 2018, I had planned about a half dozen posts to get things going at a good pace. However I backed off and put the blog on hold when I decided that I was not ready to endure some of the controversy which I might create. Recently, I decided to go forth again because I see little progress in the field of autosomal DNA matching despite all of the money being spent on consumer genetic testing. One controversy which I have been involved in over the years is short segment matching, so I am devoting this blog entry to that topic (which was my original plan).

Experts in genetic genealogy tend to have a disdain for lowering the default parameters with autosomal one-to-one comparisons. The resulting short segment matches are viewed as something to disregard. Not only are they thought to be useless, but they are labeled with the pejorative “false positive”. Fortunately I had no expert to guide me when I was faced with understanding my Family Finder (autosomal DNA) test results, so I was able to get a different perspective on the problem.

My interests were in finding more distant rather than recent ancestors. I had some luck with Y-DNA in smashing the brickwall for my patrilineal third great grandfather, but I still had one for his wife. I decided to follow up my initial Y-DNA testing with Family Finder, FTDNA’s autosomal DNA test. When I received my Family Finder test with thousands of matches, I was a bit overwhelmed – a feeling which I am sure many of you have had as well. My instincts were that to find distant matches, the matching segments would have to be smaller. I will save my story about breeching the third great grandmother’s brickwall for later if anyone is interested, but I definitely could not have achieved that without short segment matching.

Still, I was faced with the notion shared by most of the field that the parameters such as what I had used for short segment matching were so small that the matches which they produce tend to be false positives. With my rudimentary knowledge of probability and statistics, I thought that even the small segments were big enough to have statistical significance, so I was not buying the pronouncements of a high priest as to whether a segment was a false positive or not. For instance, the chance of two kits getting a half-match by chance at a given SNP pair is about seven out of eight. To get at least a half match with a hundred consecutive pairs just by chance, raise that 7/8 to the one hundred power and you get less than two out of a million chances. So much for the chances of a false positive. If you are having trouble with this, so back and reread “Genetic Genealogy’s Chain Letter Fallacy”.

Looking over what had been written about the issue, I found that the methods used by experts/professionals to “prove” that small segments tended to be false positives centered around either the child having a larger matching segment to someone else than one of his parents did in the same area or a matching segment to someone else popping up for the child, but neither parent having a similar match with that person. I had seen these situations and my intuition immediately was that both parents contributed DNA to reconstruct a segment of DNA from a common ancestor (i.e. the parents were related). Of course, the expert/professional will say that his/her parents were not related. I think that whomever makes that statement is very presumptuous. If you go back eight generations, you have 256 possible lines, half on the maternal side and half on the paternal. Can anyone definitely say that none of the 128 lines on the maternal side share a common ancestor with any of the 128 lines on the paternal? “Genetic genealogy’s Chain Letter Fallacy” says no.

I would like to present examples of how the parent’s DNA can come together for their child to have a larger matching segment than either of its parents do with the same person. I am using data for a child and its mother and father plus a cousin (4th to the child, 3rd once removed to the mother). The cousin has no close relationship to the child’s father, but I believe most people who can trace back to American Colonial times likely are distantly related several ways.

Figure 1 shows the case of a growing segment with child. Some experts have used such examples to say that the larger matching segment has to be false. I think this clearly shows how matching DNA between the father and the cousin, though the segments are not long enough to be called a match, fills in to create a longer matching segment between child and cousin than the cousin had with with the mother.

Figure 2 illustrates the case with a popping up segment.

Here the matching DNA between the mother and cousin and the father and cousin combine to create a longer match between the child and cousin.

Clearly small segments have not been proven to be false positives. The question now is how do we identify the ones which are useful in proving a match?


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