BigY "matches"

As a complete layperson when it comes to interpreting BigY matches, I rely on the people on the DNA forums, YFull.com, and the wonderful work of Alex Williamson and his BigTree (for R-P312 and sub branches). Wish there was an equivalent to Alex for I1-Z60!

This post is me thinking out loud as I explore why the yDNA67 group of matching RUNCIMANs are showing up in the BigY Match lists for the yDNA67 group of matching FAIRBAIRNs.

Back when both projects began individual SNP testing we learnt that both were I-Z60, so on the same branch back within recorded history.
They subsequently diverged.

From FTDNA's Learning Centre on

Big Y – Matching

The Big Y – Results,  Matching tab displays your Big Y matches. A person is considered a match if they have four or less differences in SNPs with you.
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where
Known SNP Difference   

This column displays the number of known SNPs within your subclade that you do not share with the specified match. The following values are possible:

• 0 – Indicates that you share all known SNPs with the specified match.
• 1 – Indicates that there is one known SNP in the subclade that you do not share with the specified match.
• 2 – Indicates that there are two known SNPs in the subclade that you do not share with the specified match.
• 3 – Indicates that there are three known SNPs in the subclade that you do not share with the specified match.
• 4 – Indicates that there are four known SNPs in the subclade that you do not share with the specified match.

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Subclade

A subclade is a subgrouping in the haplogroups of the human genetic trees. This may be either the Y-chromosome tree or the mitochondrial tree. Subclades are more specific to a location or population group than the major branches (haplogroups).

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Which I suppose explains how come the newly tested FAIRBAIRNs are showing up in the already tested RUNCIMANs matches and vice versa if you take "your subclade" back far enough, although I'm not altogether sure I consider 4000+ years before present a "match". Perhaps I'm too used to working with atDNA these days!

Both surname groups were I-Z60 back when we were doing individual SNP testing to advance down the tree, formed 4300 ybp according to YFull. 

But then the RUNCIMAN matches headed off down I-Z160 to their own newly created SNP I-Y22033 (YFull, formed 500 ybp).

We knew back at the individual SNP testing stage a while ago that the FAIRBAIRNs are I-Z140 (which YFull estimate as forming 4100 ybp)

A brief overview of some of this structure is available on DNASurnames under the I haplogroup chart which shows selected branches, the patriarch for the line, and the tested participants who agree to be so shown.  Branches can be expanded or collapsed for a better overview by clicking on the + - symbols.

Looking at the latest set of FAIRBAIRN BigY results, 5 of the 6 RUNCIMAN tests show in their match list as having 4 SNP differences PR1457 Z140 Z141 PF3832, with one not showing Z140 in their list!

Shared novel variants range from 30 to 41 for the RUNCIMAN matches, compared with 57 and 61 for the two other tested FAIRBAIRN matches with zero known SNP differences.

An interesting exercise in timeframes.

yDNA matches don't tend to appear in Autosomal match lists

BigY matches don't seem to appear in yDNA STR matches (although for this one I have noted some exceptions). 

Not at all sure what I've really learnt from here but it is fascinating watching the developments on the respective yDNA haplogroup trees at FTDNA, YFull and Alex's BigTree.