Do shared matches NOT all share the same DNA segments?

[melba_schmelba]

Given, what I found out, that many shared matches do not necessarily indicate any recent common ancestor at all between three people - what I do not understand is, where there IS actually an exact or overlapping segment match or matches between three people - why can these not be marked up as Super shared matches, Starred shared matches etc. etc. That would seem very useful, so why do they not do this? I'd say it would cause too much strain on the servers, but If I can check 20 in 1/2 hour, surely it should only take a computer a few seconds    ?
  I now really see why the DNA experts amongst us decry the lack of a chromosome browser on Ancestry, as I am sure many of us are being led a wild goose chase thinking people in shared match lists are connected when they are not.

[Janethepain]

Thanks so much for thinking that all through.  I'm still in 'sore head' mode, though I have read your posts!  I am off to Edinburgh for a weekend with my sister, away from much ancestry work - well I'll probably have a few wee peaks!

Back on the job on Monday!

[melba_schmelba]

Thanks so much for thinking that all through.  I'm still in 'sore head' mode, though I have read your posts!  I am off to Edinburgh for a weekend with my sister, away from much ancestry work - well I'll probably have a few wee peaks!

Back on the job on Monday!

It still confuses me Jane, even though I have supposedly proven how it works to myself . I think the reason why a lot of us assume all people in a shared match list are connected is because the vast majority of the time when we look at shared matches, it is for more distant links, we rarely look at shared matches for our very close relatives as there seems no need.
   But if you do that it does become quite obvious that all shared matches cannot be related to each other, as for close relatives we can easily identify that some of the shared matches come from, say the paternal great grandfather's line, and other's the line of his wife, the paternal great grandmother, who we know are not connected. But looking at shared matches for more distant links, we probably don't know how many of those shared matches link to us, or to them, so we have assumed they are probably all linked.
  So I think we all need to start bugging Ancestry, either for a super shared match feature that shows when three people all share at least part of the same segment, or for a chromosome browser as gedmatch/23andme/myheritage/familytreedna all have.

[lutrinae]

I am new to genetic genealogy too and it's making my head hurt. So far I have only managed to find a couple of matches with public trees whose trees actually have common ancestors - which is nice I guess, I know my paper trail was right in that respect.

Otherwise I am just going through high centimorgan matches, seeing who our common matches are, and sort of deducing what side of my tree they are from based on location/surnames etc (this is easier when, like me, you have some distinct areas eg one line is from Shetland) and whether they match with someone who I know to be verified from a particular line.

Not very scientific but it's a start! I haven't a clue how to break down any brick walls, as yet

[melba_schmelba]

I am new to genetic genealogy too and it's making my head hurt. So far I have only managed to find a couple of matches with public trees whose trees actually have common ancestors - which is nice I guess, I know my paper trail was right in that respect.

Otherwise I am just going through high centimorgan matches, seeing who our common matches are, and sort of deducing what side of my tree they are from based on location/surnames etc (this is easier when, like me, you have some distinct areas eg one line is from Shetland) and whether they match with someone who I know to be verified from a particular line.

Not very scientific but it's a start! I haven't a clue how to break down any brick walls, as yet

  I agree it helps to have distinct areas tracing wise - only a small part of my tree is Irish so I know who all my Irish matches likely link up to. Of course actually proving those Irish lines is much more difficult than in England, although it's definitely getting better and more registers are being put online.
I definitely wouldn't say to dismiss shared matches full stop, obviously they can be very useful. But for lower matches i.e. below 35 cM, you need to be aware that the chance of the fact three people all match may then be coincidental increases the closer you get to 6 cM.
  If all three people are known to have a good % of their tree that lived in the same area for generations, the chances of all three connecting coincidentally, and not actually having exact segment(s) in common between the three greatly increases I suspect.

[melba_schmelba]

Doing some more shared match hopping, it's becoming clear to me that the 20 centimorgan minimum criteria applies BOTH ways - so YOU have to match a shared match at a minimum of 20 centimorgans, and also the match at the top, ALSO has to match that shared match at 20 centimorgans or more or the shared match won't show up. But those matches may be on completely different segments. And this is regardless of how much you match the match at the top...confused?! I am !!! …..

[sugarfizzle]

Melba, I don't think you are correct in saying 'YOU have to match a shared match at a minimum of 20 centimorgans, and also the match at the top, ALSO has to match that shared match at 20 centimorgans or more or the shared match won't show up.'

I have several examples of shared matches sharing less than 20 cMs with either me or a decent match.

I don't know their exact criteria, but it seems to be if two of the shared matches are closely related, then they will sometimes show up with a third match, even if they share less than 20 cMs.

For example, me and my first cousin share 723 cMs.
She matches with someone at 25 cMs, I match them at 15.2 cMs.
We show as shared matches with each other.

My third cousin and I share 153 cMs. Her daughter and five of her daughters (third cousin's grandaughters) have also tested. One of the grandaughters shares 7.7 cMs with me.

Shared matches for third cousin, this grandaughter doesn't appear, though the other 5 family members do, plus several more matches.
Shared matches for the grandaughter 7.7, the whole family of 6 appear as shared matches, plus a couple* more descended from the same branch as the third cousin, also a couple more.

Looking at the same family from my first cousin's match list.

Third cousin shares 63 cMs with my cousin, considerably less than me.
Shared matches for third cousin, only 3 family members show up, three grandaughters don't appear.
My cousin is not a match at all with one of them. One other shares 8.3 cMs.
Shared matches for grandaughter 8.3, the whole family appear, less the missing match, plus the same couple*

Ancestry are really a law unto themselves. Without a chromosome browser there is no way of sorting out matches properly. Recent improvements (with the exception of Thrulines) have been very helpful, but combined with a browser they would be exceptionally helpful.

Regards Margaret

[melba_schmelba]

Melba, I don't think you are correct in saying 'YOU have to match a shared match at a minimum of 20 centimorgans, and also the match at the top, ALSO has to match that shared match at 20 centimorgans or more or the shared match won't show up.'

Hi Margaret , thanks for your input. I know it is unbelievably confusing but I think what I said is compatible with your examples, I will attempt to explain below

I have several examples of shared matches sharing less than 20 cMs with either me or a decent match.

I don't know their exact criteria, but it seems to be if two of the shared matches are closely related, then they will sometimes show up with a third match, even if they share less than 20 cMs.

For example, me and my first cousin share 723 cMs.
She matches with someone at 25 cMs, I match them at 15.2 cMs.
We show as shared matches with each other.

If you look via your first cousin's account, with the match to this person at 25 cM, you will show in the shared matches, as will this person as you both match your 1st cousin above 20cM. However, if you look via your account, it is not possible that this other person will show in shared matches between you and your first cousin - no matches are shown in shared match lists below 20 cM.

My third cousin and I share 153 cMs. Her daughter and five of her daughters (third cousin's grandaughters) have also tested. One of the grandaughters shares 7.7 cMs with me.

Shared matches for third cousin, this grandaughter doesn't appear, though the other 5 family members do, plus several more matches.
Shared matches for the grandaughter 7.7, the whole family of 6 appear as shared matches, plus a couple* more descended from the same branch as the third cousin, also a couple more.

If you are talking about accessing via your DNA  account, it is correct that the granddaughter who matches you at 7.7 cM will not show in a shared match list with your third cousin. It is also correct that if you look at the granddaughter's profile, the whole family of 6 will show in the shared matches - as they all match the granddaughter at a level much higher than 20 cM. As I said in my original post, the level of cM connection between you (or whoever's DNA account you are accessing) and the match 'A' at the top, is not a factor, it is only how much 'A' matches shared matches B, C, D, E, F etc.
 

Looking at the same family from my first cousin's match list.

Third cousin shares 63 cMs with my cousin, considerably less than me.
Shared matches for third cousin, only 3 family members show up, three grandaughters don't appear.
My cousin is not a match at all with one of them. One other shares 8.3 cMs.
Shared matches for grandaughter 8.3, the whole family appear, less the missing match, plus the same couple*

Similarly as above, the shared matches between the 8.3 granddaughter will show people that are closely related to her, as they all match her at 20 cM or above, anyone that matches the granddaughter at less than 20cM will not show up.

Ancestry are really a law unto themselves. Without a chromosome browser there is no way of sorting out matches properly. Recent improvements (with the exception of Thrulines) have been very helpful, but combined with a browser they would be exceptionally helpful.

Regards Margaret

I think the rules are as I have discovered, that for people to show on a shared match list:

Both you (or the DNA account you are accessing) 'X' AND the match 'A' must match all shared matches at a minimum of 20 centimorgans.

This is regardless of how much 'X' matches the match 'A'. It also is regardless of where 'X' or 'A' match any of the shared matches - so 'X' and 'A' may match the shared matches on completely different segments. So many people on a shared match list may share no common ancestry.

Given that, I 100% agree with you that a chromosome browser is a must as a lot of people will be making false assumptions otherwise that people on a shared match list are all linked when they very likely are not  .

[melba_schmelba]

I have just become aware of the segment search facility on GEDMATCH Genesis premium Tier 1 tools ($10/month). This seems to be the only current way to automatically find all your matches that match segments exactly between each other (rather than the shared matches that can match anywhere) - it groups your top  3000 matches by segments in common - Mercedes Brons has written a detailed blog on this:

http://whoareyoumadeof.com/blog/2017/11/18/how-to-use-the-gedmatch-matching-segment-search/

In her case, in studying her husband's DNA, she found a small group of matches that all matched on a certain segment all had English/Irish sounding names which was odd as her husband is Mexican, of mostly native ancestry so she was able to work out this segment must have come from his Irish ancestor - and if those matches have GEDCOM trees attached that is useful to determine what the exact connection is.