Ancestry Matches 7cM and less

It's not so much a question of how they're used, it's more a question of how the evidence was compiled. Bryman has hit the nail on the head.

Also, don't be too ready to label short DNA matches as spurious - Ancestry's White Paper esrtimates that 50% of 6cM matches and 70% of 7cM matching segments are genuine. It stands to reason that a match is far more likely to be genuine if both parties, working completely independently, have traced their ancestry to the same couple, as in my case.

If the matches were spurious, ie random, they would surely be just as likely to occur between people who aren't known to be related?

Your faith in the record keepers and in the integrity of your ancestors is creditable, but even if you're 95% certain about each generation, by the time you get back 9 generations there's a 37% chance of an error somewhere along the way.

 

Neither is used to validate the other - both sets of evidence stand on their own in terms of proving that two individuals are related. There is no circular argument.

Remember, I'm not trying to prove that a particular DNA segment came from a specific ancestor (or pair of ancestors) - I'm simply trying to prove beyond reasonable doubt that I'm descended from that pair of ancestors. This will be just as likely as it was before if the DNA match is spurious, but far more likely if the DNA match is genuine.

I assume you are quoting from someone else - whoever wrote that is clearly not familiar with Bayes theorem.

Finally, just a reminder that in the case I described in my newsletter I wasn't talking about a single match with a single putative cousin, but matches between me, my brother, and our 2nd cousin with three Americans who, though quite distantly related to each other, can all trace their ancestry back to John Medley, the son of Francis Medley and Mary Linnett.

EDIT: we don't all match with all of them, but all of us match with some of them.

 

That's one of the reasons why we take DNA tests - to knock down 'brick walls' and find the connections that we don't know about. Contrary to what you are implying, the DNA evidence isn't contradicted what's in your tree and in your brother-in-law's tree, it's merely unproven.

Instead of throwing away the evidence, surely you should be filing it away for possible future use?

 

Yes, it certainly makes it seem more likely that they are just random when you put it that way. But then it assumes that you're not related to your husband, whereas the only question is how closely-related the two of you are (on average we're all 10th cousins or thereabouts).

 

As I understand it, unless there is a segment of 8cM or more there will be no match. So add a note to those entries just in case.

 

I'm sure the bloggers are wrong but I'll ask - even if they're right it could only apply to existing matches, not future matches. In the meantime I'm adding notes to all matches of interest whenever the average segment length is below 8cM. Perhaps the more important question for those of us who are managing multiple tests is precisely when in August the change is likely to go into effect.

Again, I think they're wrong but will check. Since the change was announced Ancestry have only been displaying rounded figures.

 

I'll be interested to see how this figure changes after it moves from 6 to 8

 

I suspect you'll still have twice as many matches as me

 

That sounds very strange. It might be a result of an earlier update splitting what had previously been considered a single segment.

 

Unfortunately this is all I have close to me, maybe you have more?


My closest match has:
132 cM across 7 segments,
127 cM across 7 segments
125 cM across 9 segments
119 cM across 4 segments
109 cM across 4 segments

and then I get into 4th cousin matches
49 cM across 5 segments

And 2 of my closest matches have no trees.

The 119 cM across 4 segments really interests me, as he has my surname and I have no idea where he fits it. Maybe why he doesn't reply to me?
Ancestry is suggesting he'd quite close.

 

I have a few more (but not that many after deducting all the ones who tested at my request!).

But bear in mind that 4th cousins only share 14cM on average so most won't show up as a close match at Ancestry, and nor will some 3rd cousins and 2nd cousins once removed - so many of the most useful matches will be hidden amongst the distant matches. In fact, unless you have a very recent 'brick wall' close matches aren't a lot of use - the exception would be if you can persuade your cousin to give you access to her matches..