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Family Finder Advanced Topics Advanced discussion about Family Tree DNA's Family Finder Product.

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  #1  
Old 16th July 2017, 11:29 PM
mabaker mabaker is offline
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Match all on one chromesome

Using Family Finder (Autosomnal DNA), one of the tests I mananage has a match with 71 shared centimorgens and a longest block of 48. Using the chromosome browser, it appears that the match is all one long block on the 9th chromosome. If I understand things correctly, this would suggest that the two people tested are 2nd or 3rd cousins. However, comparing the family trees, it appears that they are 5th cousins once removed. Is it coincidence that they match so well and only on one chromosome, or could there be something else going on here? (I have scanned their family tress and do not see any other shared relatives other than the line that makes them 5th cousins once removed.)
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Old 17th July 2017, 10:47 AM
John McCoy John McCoy is offline
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Yes, it is entirely possible to match a 5th cousin with a long segment (48 cM is what you mentioned), but it doesn't happen very often. The prevailing opinion right now is that about half of 4th cousins won't be detected as a match, so we would predict that the odds of a strong match for a 5th cousin once removed would be considerably lower. If it were me, I would be looking for other people who match both kits on the same segment (or a significant part of it) to see if they share the same common ancestor (I rely on the tools at the free GEDmatch site for that sort of analysis). I have found one or two cases like that. When you have several people who descend from the same common ancestor but through different lines of descent, that tends to validate your conclusion that the genetic evidence is real, and also that your paper trail is correct.
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Old 17th July 2017, 11:25 AM
georgian1950 georgian1950 is offline
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Quote:
Originally Posted by mabaker View Post
(I have scanned their family tress and do not see any other shared relatives other than the line that makes them 5th cousins once removed.)
You cannot assume that shared relative is the one responsible for the big segment.

Some genetic genealogists believe there is a source of common ancestry in early Colonial Virginia or North Carolina, but they do not seem to be eager to find the source. It is there and it is huge. I have found that these big distant segments are formed occasionally when both parents of the matches have multiple lines back to this common ancestor. You are only chasing a phantom with these long, distant segments. You have better places to put your efforts.

Jack Wyatt

Last edited by georgian1950; 17th July 2017 at 11:27 AM. Reason: punctuation
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Old 17th July 2017, 01:27 PM
mattn mattn is offline
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Talking

Quote:
Originally Posted by mabaker View Post
Using Family Finder (Autosomnal DNA), one of the tests I mananage has a match with 71 shared centimorgens and a longest block of 48. Using the chromosome browser, it appears that the match is all one long block on the 9th chromosome. If I understand things correctly, this would suggest that the two people tested are 2nd or 3rd cousins. However, comparing the family trees, it appears that they are 5th cousins once removed. Is it coincidence that they match so well and only on one chromosome, or could there be something else going on here? (I have scanned their family tress and do not see any other shared relatives other than the line that makes them 5th cousins once removed.)
My grandma has a 4th cousin once removed who matches 71 cM's with a longest block of 57 so it happens. The predictions are usually overly optimistic especially if only one segment is involved.
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Old 17th July 2017, 01:29 PM
mattn mattn is offline
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Quote:
Originally Posted by georgian1950 View Post
You cannot assume that shared relative is the one responsible for the big segment.

Some genetic genealogists believe there is a source of common ancestry in early Colonial Virginia or North Carolina, but they do not seem to be eager to find the source. It is there and it is huge. I have found that these big distant segments are formed occasionally when both parents of the matches have multiple lines back to this common ancestor. You are only chasing a phantom with these long, distant segments. You have better places to put your efforts.

Jack Wyatt
Talk about chasing phantoms.....
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  #6  
Old 17th July 2017, 02:22 PM
georgian1950 georgian1950 is offline
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Quote:
Originally Posted by mattn View Post
Talk about chasing phantoms.....
Mattn, take a GEDmatch 'X one-to-many' list using default parameters and run the 'X one-to-one' comparisons to get the matching segments. The 'x one-to-many' uses 500 SNP's and 7.0 cM minumum segment size. When you select the 'X one-to-one' from the list, it is uses 700 SNP's as the default there, so you have to plug in the 500 SNP's to be sure to get the segments. Look for trianguations with the matching segments. After doing a few dozen or so, it is hard not to conclude that they all point to a common ancestor.

If as a male you only have a handful of matches on the list, use a list for a female in this exercise.

Jack

Last edited by georgian1950; 17th July 2017 at 02:25 PM.
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