Ancestry shared matches and 'relatives in common'

[Pauline]

When I first got my DNA results I tended to assume that if an unidentified match showed up as a shared match with a documented cousin, the unidentified match probably belonged in the same part of my tree.

Now, in the light of experience, I'm wondering just how safe this assumption might be, particularly when only one shared match shows up. More and more I'm finding occasions when a shared match is actually down to 'relatives in common', rather than all 3 of us having shared ancestry.

Sometimes this is easy to spot because an unidentified match has shared matches with known cousins in two (or more) unrelated parts of my tree. However, if there is just the one shared match it may not be apparent, and has the potential to mislead.

It's at times like this when I feel a chromosome browser would be a useful addition at Ancestry.

Have other people come across this problem much? Is it something that becomes more apparent as we gain more 'close cousin' matches?

 

[Bryman]

Sorry if my ignorance is showing here but . . .

More and more I'm finding occasions when a shared match is actually down to 'relatives in common', rather than all 3 of us having shared ancestry.

Presumably, any instances of individuals having 'relatives in common' could be shown to have shared ancestry if enough (just a few more) generations could be traced back, so not completely misleading necessarily. I try to identify parts of my tree that potential cousins belong to based on surnames of my eight great grandparents and so far this has not led to any howlers. A couple of times I have found shared matches from more than just a single part of my tree but that is unusual and may indicate a possible cross link between those parts of my tree.

It's at times like this when I feel a chromosome browser would be a useful addition at Ancestry.

I have not used a chromosome browser and am not sure what I could gain from using one. However, if you believe that such could be of sufficient benefit to you, why not export your Ancestry results to another site which does provide a browser and interrogate the results there? Would you also need access to results of potential cousins at that other site?

 

[peter]

Have other people come across this problem much? Is it something that becomes more apparent as we gain more 'close cousin' matches?

I've come across one case of a genetic cousin who matches two of my cousins but in different parts of my tree. It's presumably because of the likelihood of wrong assumptions being made that Ancestry only show shared matches of 20cM or more - users in the US can have thousands, or even tens of thousands of close matches.

It's at times like this when I feel a chromosome browser would be a useful addition at Ancestry.

The chance of a chromosome browser helping is pretty minimal. It never actually helped me in the dark days of FTDNA and GEDmatch, it was just another theoretically useful but practically useless tool.

 

[Pauline]

Presumably, any instances of individuals having 'relatives in common' could be shown to have shared ancestry if enough (just a few more) generations could be traced back, so not completely misleading necessarily. I try to identify parts of my tree that potential cousins belong to based on surnames of my eight great grandparents and so far this has not led to any howlers. A couple of times I have found shared matches from more than just a single part of my tree but that is unusual and may indicate a possible cross link between those parts of my tree.

The thing about 'relatives in common' is that there isn't any shared ancestry further back - beyond the possibility that we are all related if we go far enough back.

So I may share ancestry with, say, John and Jane, and Jane shares ancestry with John, but that doesn't necessarily mean that all 3 of us share a common ancestor. And if we don't it doesn't necessarily indicate a cross link between different parts of my tree either.

I have not used a chromosome browser and am not sure what I could gain from using one. However, if you believe that such could be of sufficient benefit to you, why not export your Ancestry results to another site which does provide a browser and interrogate the results there? Would you also need access to results of potential cousins at that other site?

The advantage of a chromosome browser in this particular situation is that you would be better able to see if all 3 of you share the same DNA or not.

I have exported my DNA to other sites which have a chromosome browser but that only helps if the others matches involved have also uploaded their DNA there. Some people are simply not interested in doing so or don't respond to contact.

 

[Pauline]

The chance of a chromosome browser helping is pretty minimal. It never actually helped me in the dark days of FTDNA and GEDmatch, it was just another theoretically useful but practically useless tool.

I agree that a chromosome browser may sometimes be of limited use , but I think this is one of those areas where one could be really useful. If I have match who has shared matches with documented cousins in completely different parts of my tree then I know to consider the possibility of 'relatives in common'. However, if there is just the one shared match then I have no way of knowing if all 3 of us share a common ancestor or not.

Sometimes this may not matter that much but if I am looking for matches to help confirm a particular link in my tree, or to help in breaking down a brick wall, then it may matter a lot.

I've come across one case of a genetic cousin who matches two of my cousins but in different parts of my tree.

I have found quite a few cases among my matches which is why I raised the question.

 

[Pauline]

It's presumably because of the likelihood of wrong assumptions being made that Ancestry only show shared matches of 20cM or more

Yes, I would guess so, but it can be really frustrating at times. Another thing that would be helpful is to know how much DNA shared matches share with each other. I know shared DNA can vary hugely but having some idea of how closely an unidentified match and a known cousin might be, can sometimes really help in working out how the unidentified match is related to me.

 

[peter]

I agree that a chromosome browser may sometimes be of limited use , but I think this is one of those areas where one could be really useful. If I have match who has shared matches with documented cousins in completely different parts of my tree then I know to consider the possibility of 'relatives in common'. However, if there is just the one shared match then I have no way of knowing if all 3 of us share a common ancestor or not.

Even if you share a segment of DNA in the same position on the same chromosome it doesn't guarantee that you all share a common ancestor, and if you don't share the same segment it certainly doesn't mean that you don't share a common ancestor.

It's nuances like these that have persuaded me that 99.9% of family historians would do well to stay clear of genetic genealogy blogs and tools.

 

[peter]

Yes, I would guess so, but it can be really frustrating at times. Another thing that would be helpful is to know how much DNA shared matches share with each other. I know shared DNA can vary hugely but having some idea of how closely an unidentified match and a known cousin might be, can sometimes really help in working out how the unidentified match is related to me.

I agree - that's why I get access to the DNA matches of as many of my known cousins as possible.

 

[Pauline]

Even if you share a segment of DNA in the same position on the same chromosome it doesn't guarantee that you all share a common ancestor, and if you don't share the same segment it certainly doesn't mean that you don't share a common ancestor.

Yes, I appreciate that, but with closer cousins with whom you share several segments it may be more helpful.

I guess it is a case of having a range of tools at our disposal to help with those more problematic matches. The shared match feature alone has too many limitations.

that's why I get access to the DNA matches of as many of my known cousins as possible.

I haven't managed to get anyone to agree to give me access as yet, although admittedly I haven't asked that many. One issue for me is that if someone allows me to view their matches, I couldn't then refuse to reciprocate, and I'm not quite sure how I feel about that, particularly as they would be able to see all the notes I've made.

 

[peter]

The shared match feature alone has too many limitations.

The limitations are there for a good reason - there would be so many mistakes made by unwitting that it would bring Ancestry DNA into disrepute.

particularly as they would be able to see all the notes I've made

I don't think a Viewer can see notes - perhaps someone can check as I have a meeting to go to?

 

[Pauline]

I don't think a Viewer can see notes - perhaps someone can check as I have a meeting to go to?

See Tim's post here

 

[Pauline]

The limitations are there for a good reason - there would be so many mistakes made by unwitting that it would bring Ancestry DNA into disrepute.

Hmm. Ancestry don't seem too worried about their reputation when it comes to things like Tree Hints, and all the mistakes that they lead to.

 

[peter]

Hmm. Ancestry don't seem too worried about their reputation when it comes to things like Tree Hints, and all the mistakes that they lead to.

I'm not aware that Tree Hints have damaged Ancestry's reputation.

 

[PhilGee]

I'm not aware that Tree Hints have damaged Ancestry's reputation.

In this case that's because, unusually, people do not "shoot the messenger" touting the rubbish of others.

However, I am at a loss as to why Ancestry providing "accurate" information that two samples have matching segments that are not in their timber/pile-up database should damage their reputation. It is the application of this information that could be erroneous - and probably no where near as "idiotic" as hints.

 

[canadianbeth]

Ancestry sent me a hint for a first cousin once removed that was incorrect. It was the 1911 census and her name and apparent birth date were correct, but when I clicked on it saw that her parents and siblings did not match what I already have, a connection to the Barratt branch. The same page had other hints which did match. Given that it was a common-enough name - Thomas - and in Wales, I have to assume that there was another Florence Thomas living in Cardiff at the time. She is in the 1901 Wales census with just her father. She was also married in 1908. I have not found her in the 1911 Wales or England census.

 

[peter]

However, I am at a loss as to why Ancestry providing "accurate" information that two samples have matching segments that are not in their timber/pile-up database should damage their reputation. It is the application of this information that could be erroneous - and probably no where near as "idiotic" as hints.

What is the relevance of pile-ups?

 

[PhilGee]

Precisely.

 

[peter]

Precisely.

Precisely what?

 

[PhilGee]

What is the relevance of pile-ups?

Regarding identifying cousins, very little at best.

 

[peter]

Regarding identifying cousins, very little at best.

That's a very cryptic comment. Please explain exactly what you meant by this sentence in your earlier post - I can only guess at what you intended it to mean and none of the interpretations I've come up with so far make sense to me:

However, I am at a loss as to why Ancestry providing "accurate" information that two samples have matching segments that are not in their timber/pile-up database should damage their reputation.