As genetic genealogy testing consumers we must rely on the data analysis the companies are providing us with. Companies like AncestryDNA are putting out statistics like these:
To me a 99% probability is good? This would imply that my Aunt's 5th cousin match sharing a 20 and 15.7 cM segment more than likely shares IBD segments? The 99% probability means they likely share an ancestor within the past 6 generations. If we take away 10 cM's for phasing, timber etc. we are still in the 99% probability range.
If we find a triangulation on one of these segments, with good overlap, and that happened to be a 5th cousin match, I don't understand why that would be suspect either? As some would say. They would agree with this statement by Ancestry "In populations that have grown rapidly in the past 200 to 300 years, individuals are more likely to be related to each other through two or more ancestral couples. Such population growth may also lead to marriages between related individuals, such as second cousins. As in the founder effect, this also leads to an excess of DNA sharing, but due to multiple common ancestors living one to two hundred years ago, rather than thousands of years ago." The triangulation naysayers would say how can you tell where such a segment comes from if you might share several ancestral lines? Maybe you can use segment mapping or build out your tree as far as possible? Some are also saying that segments shared between those with many Colonial American lines are the result of endogamy. Read Ancestry's statement again. They are saying they aren't seeing the founders effect?
At AncestryDNA the lack of segment data makes it impossible for us to even try to decide which lines, out of maybe a couple possibilities, we might match on. You don't have to be a scientist to map segments.
I think most of the experts in the genetic genealogy field would agree with this statement from Ancestry:"The longer the stretches of evidence for identical haplotypes the more evidence there is that the identity is due to a recent common ancestor."
From Ancestry "Our test set contains over 150 genotyped samples from a large family with a well-researched pedigree containing about 2,500 relationships that vary from 1 meiosis to 15 meioses. In order to estimate recall, we must know whether a given pair of individuals has IBD." Ancestry compared 150 samples to find IBD, so we can find IBD by comparison. Wonder if they had Colonial American ancestors? I realize they only used this test set for finding IBD in close relatives, but this would seem to suggest mapping would help in establishing a relationship with more distant relatives.
We need the studies carried out by Ancestry and the other companies to guide us as to whether our matches have a good chance of sharing ancestors within the past 6 generations. We need good statistics regarding the likelihood a segment is IBD. Personally if a segment has a good chance of being IBD and I share a set of common ancestors with a match I have little doubt where the segment came from.
Reading Ancestry's white paper for matches did give me some pause for thought...
"third cousins, in fact, are only about 98% likely to have any IBD." Only? That's a pretty high percentage to me? That statement is a little troubling. I hope the rest of the reasoning behind their analysis is better?
Given the chance I think non scientists, provided with segment IBD probabilities and good trees, can make valid connections with matches