12-marker tests are good for ruling out a relationship. If two males
with the same surname do not match on at least 11/12 markers, it is
highly unlikely that they are related in a genealogical time frame.
If two males do match on 12/12 markers, that still does not guarantee
they are related. They could be on different branches of the Y-
phylogenetic tree and not have had a common ancestor for thousands of
years. For example, a male in haplogroup J1 can match a male in
haplogroup J2 on 12 markers; this happens among branches of haplogroup
E1b1b1 and R1b also. Even if they are on the same branch, the most
recent common ancestor between two males who match on only 12 markers
could have lived several thousand years ago. 12 markers can shed
light on deep ancestry, provided that there is a good prediction or
confirmation of the haplogroup. But I would not recommend using 12
markers to learn about recent ancestry. Even 25 markers do not give
enough resolution in many cases to estimate the time to most recent
common ancestor among Y-DNA matches.
Also, as more SNPs (the single nucleotide polymorphisms that define
sub-branches of the phylogenetic tree) continue to be discovered, it
is necessary to test out to the tiniest branches that continue to be
discovered. I have noticed that for haplogroups such as E1b1b1 and
R1b that 25-marker matches can be on different branches and therefore
not have a common ancestor in a genealogical time frame.
Generally, I prefer members of my DNA projects to test to 67 markers,
and I recommend that at least one member of each cluster of close
matches has a deep clade test to verify which branch of the
phylogenetic tree they are on.
On Sat, Jun 12, 2010 at 8:12 AM, Jeff Malka
Does a 12 marker match mean *anything* at all? And if not, why bother doingthem at all?