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The Role of "In-Betweeners" in the Analysis of Y-DNA Test Results
One aim of genetic genealogy is to determine whether or not individuals are genetically related within "genealogical time."  Sometimes, people will appear unrelated, until someone else genetically "bridges the gap" between them.  Family Tree DNA calls these individuals "in-betweeners," and they can be important in cases where a genetic match is otherwise "borderline" or even unsuspected.

Below is a real example from a project at FamilyTreeDNA, a project which I have found myself frequently in disagreement with when it comes to its being "over-zealous" in connecting people on what I consider insufficient evidence.  In this case, my family is the one dubbed "Yellow," and the project is alleging a connection between it and the "Orange" family, with "Green" being the in-betweener that unites them.  Unfortunately, only Yellow has been tested to 37 markers because if all three were tested to 37 markers, the issue of a relationship would doubtless disappear (as the distance between them widens).

I show the three below in comparison to the top two Haplogroup R1b modal haplotypes.  This haplogroup is the most common one in western Europe, with 70 to 80+% of males in the British Isles being R1b and these are Scots/English families.  I point this out to emphasize that it is their differences from the modal values that count.  Being modal here simply makes you related to tens of thousands of other R1b's.

Please also note the FamilyTreeDNA Guidelines for matches at 25 markers, which indicate that genetic distances (GD) of four or more are a non-match, unless there is an "in-betweener" to bring them together.  As you can see below, each of the families has only a 20/25 or 21/25 match with either of the other two and none is an in-betweener.  In other words, the three families are not related, not within "genealogical time." 

Unfortunately, this project is asserting that they are related, supposedly by reason of the fact that Green is an in-betweener, which simply isn't the case.  For someone to function as an in-betweener, they must be close to both of the other two individuals, and Green is not close to either one.  So what would constitute an in-betweener for these groups?  (Please see the second table.)

Three Unrelated Individuals
Subject Haplotype
Markers 1-12 Markers 13-25
3
9
3
3
9
0
19
/
3
9
4
3
9
1
a
|
3
8
5
b
|
3
8
5
4
2
6
3
8
8
4
3
9
i
|
3
8
9
3
9
2
ii
|
3
8
9
4
5
8
a
|
4
5
9
b
|
4
5
9
4
5
5
4
5
4
4
4
7
4
3
7
4
4
8
4
4
9
a
|
4
6
4
b
|
4
6
4
c
|
4
6
4
d
|
4
6
4
R1b Modals 55% 13 24 14 11 11 14 12 12 12 13 13 29 17 9 10 11 11 25 15 19 29 15 15 17 17
R1b Modals 45% 13 24 14 10 11 14 12 12 12 13 13 29 17 9 10 11 11 25 15 19 29 15 15 17 17
     
Orange 13 24 14 10 11 14 12 12 12 13 13 30 17 9 10 11 11 25 15 19 31 15 15 17 17
Green 13 24 14 10 11 14 12 12 13 13 13 29 17 9 10 11 11 25 15 19 29 15 15 17 18
Yellow 13 24 14 10 11 14 12 12 12 13 13 29 17 9 10 11 11 25 16 19 29 15 15 16 17
In the table below, I've turned Green into an in-betweener.  While Orange and Yellow have a GD of five, each has a GD of only two or three from Green.  A 22/25 or 23/25 match is still not great, but at least a near common ancestor becomes a possibility.
Two Seemingly Distant Individuals United by an In-Betweener
Subject Haplotype
Markers 1-12 Markers 13-25
3
9
3
3
9
0
19
/
3
9
4
3
9
1
a
|
3
8
5
b
|
3
8
5
4
2
6
3
8
8
4
3
9
i
|
3
8
9
3
9
2
ii
|
3
8
9
4
5
8
a
|
4
5
9
b
|
4
5
9
4
5
5
4
5
4
4
4
7
4
3
7
4
4
8
4
4
9
a
|
4
6
4
b
|
4
6
4
c
|
4
6
4
d
|
4
6
4
R1b Modals 55% 13 24 14 11 11 14 12 12 12 13 13 29 17 9 10 11 11 25 15 19 29 15 15 17 17
R1b Modals 45% 13 24 14 10 11 14 12 12 12 13 13 29 17 9 10 11 11 25 15 19 29 15 15 17 17
     
Orange 13 24 14 10 11 14 12 12 12 13 13 30 17 9 10 11 11 25 15 19 31 15 15 17 17
Green 13 24 14 10 11 14 12 12 12 13 13 29 17 9 10 11 11 25 16 19 31 15 15 17 17
Yellow 13 24 14 10 11 14 12 12 12 13 13 29 17 9 10 11 11 25 16 19 29 15 15 16 17
In-betweeners do play a role in uniting otherwise distant family members, but they have to be truly in between, as "green" in the above table is.

The other bottom line here is that 25 markers should not be used to support relationships, at all, unless the haplotype is rare and there is other corroborating evidence.  You can use 12 markers to disprove a relationship, but I've seen a 23/25 match in R1b drop to 28/37.  You have to test 37 markers in R1b.  Unfortunately, this particular project standardized on 25.

Update (2015):  I would no longer consider 37 adequate in Hg R1b, so I've now standardized on 67 markers.  In some cases, I've had to go to 111 to make the determination with confidence.


 
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