Borland Genetics

I have a match on Ancestry and it says we don’t share any matches.   How could that be? I see this question come up several times a month in the various Facebook genealogy groups.   Well, there are four scenarios that can give rise to this situation, so let’s go through each of them. 1.       Ancestry sets a threshold of 20 cM for displaying shared matches.   That is, the shared match would have to share at least 20 cM with you and 20 cM with your match to be displayed as a shared match.   So if you’re down in the weeds of your matches and looking to find shared matches with donors who only match you at 10 cM, it’s possible you might find some, but what’s also pretty likely is that the segment that you inherited from your common ancestor is smaller than 20 cM, and while there may be many donors that match both you and your match on that segment, none of them match you at 20 cM (and most probably don't match your match to that...

If I match Cousin A and Cousin B on chromosome 6, does that mean that we share a common ancestor? The answer is a bit complicated! First, the general answer: General rule:  If you and cousin A and cousin B all match across a significantly overlapping span of the same copy of the same chromosome (called a segment), it is highly probable that you do so because the three of you all inherited that segment from a common ancestor. But what does that mean? First qualification :  It's not enough that the matches both match you on the same chromosome.  The matches must also match you over the same span of that chromosome.  Position on a chromosome is generally measured in MBP or Mega-Base-Pairs.  If Cousin A matches you on chromosome 6, but over the span of 23 MBP to 51 MBP, but Cousin B matches you over the span of 63 MBP to 81 MBP, this is not evidence that you and both matches share a common ancestor because these are segments to do not overlap....