Steve,

Family Tree DNA (FTDNA) provides the "male Y chromosome donor" with a "tool" 
that they call the "Tip Report."  That "calculation" (based on an algorithm 
that was developed by FTDNA) can be run to compare any two "closely matching 
donors" to see what the "odds"/"percentages" are that the two men in 
question share a "common male ancestor with the same surname" at various 
"generations back."  On the Davidson/Davison/Davisson DNA testing project 
website, we use the "rule of thumb" that any two donors being compared 
should have around 80% or better "odds" at "12 generations back," and also 
about 90% "odds" or better at "16 generations back," of sharing a common 
ancestor before we typically consider those two male DNA donors to be 
members of the same family.

Note: If we believe that all of us "trace back" to "two original people" 
(commonly called Adam and Eve), then we are ALL related.  It is just a 
matter of "how far back is too far" to be of any real value to genealogy. 
Luckily, DNA "markers" mutate over time.  If the "markers" did not mutate, 
then I presume that all humans might have the same DNA, and it would be of 
no value to genealogy.  Because of the mutations, however, trying to find 
"the most recent common male ancestor with the same surname" when that 
common ancestor lived 800-1,000 years ago (or even further back in time) 
becomes unreliable when using DNA.  Even where there was, in fact, a "most 
recent common male ancestor with the same surname" between the two men in 
question 1,000+ years ago, there would probably be so many markers that 
would not match after all of that time (thanks to mutations), that the DNA 
would not reflect the "relationship."  DNA works great, however, when trying 
to find your ancestors who lived, for example, in the 1500s-1900s.

The Y chromosome "markers" are comprised of both "slow/seldom mutators" and 
"fast/frequent mutators."  The latter often do not match between two donors 
who share an ancestor as recently as just a couple of genearations ago....so 
that is why "known close relatives" may have only 35 out of 37 markers that 
match, for example (where the two makers that do not match are 
"fast/frequent mutators"....such mutations are actually normal and 
expected).  The "slow/seldom mutators," however, typically would be the same 
between any two donors whose common ancestor lived within the last 100-500 
years or so.  The above Tip Report at FTDNA takes all of this into 
consideration when determining the "odds" mentioned above.  Without the Tip 
Report, or without input from a person who is truly knowledgable in the 
science of DNA, comparing the results between two or more people can be 
tough to do.  Most DNA testing companies, however, as well as some of the 
"stand-alone" websites that are devoted to a particular surname DNA study, 
have people who can "make sense of it all" for their donors.

Note: The above Tip Report is not "perfect," and the science of DNA seems to 
evolve with each passing year.  As an example, I have one "matching donor" 
(out of several "matches") where the typical/standard "FTDNA computer 
analysis" for that one donor shows that there is the equivalent of five 
markers that do not match between us (called a "Genetic Distance" ("GD") of 
5).  Despite that, when analyzed more closely by an expert (versus by just 
the "standard computer analysis"), the true "equivalent GD" is actually 
closer to 1 (this is a somewhat unusual case, and I will not attempt to get 
into the details here).  FTDNA is aware of these occasional anomalies, and I 
understand that they plan to update some of their standard techniques (as 
are, no doubt, many of the other DNA testing companies).  DNA testing is 
still a tremendous tool for genealogy, and I would not take anything for the 
Y chromosome DNA test results for me and for my mother's male cousin, as 
provided by FTDNA.  Both of those DNA tests provided me with information 
that was simply not available through "conventional documentation research" 
alone.

Bill Davidson 

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