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Understanding DNA Results
 An individual's results will be a series of numbers called alleles.  The markers (categories) for the alleles are named DYS (
DNA Y-chromosome Strand) These numbers will be compared to others in our group and in the Family Tree DNA database for matches.   The matches will help eliminate non-relations and confirm relations if there is a common ancestor.  They will not tell us which generation the most recent common ancestor occurs. 

The 25 match is a little more precise, but still does not tell you exactly which generation. 

For example, if all 25/25 markers match, there is a 50% probability that the most recent common ancestor was no longer than 7 generations, and a 90% probability that the most recent common ancestor was within the last 98.8 generations.  A perfect match on the 12 marker test 12/12 would show that there was a 50% probability that the most recent common ancestor was within 14.5 generations and a 90% probability that the most recent common ancestor was within 48 generations.

Please see the Family Tree DNA site  for more precise explanations.  My DNA knowledge is grossly limited.  You can learn a lot by going to this site and visiting other surname pages.

First some definitions:
Allelles: a number/position on the Y Chromosome.  These are the individual numbers (in a series) that represent your test results. 

DYS:   for DNA Y chromosome Strain

Loci:  location/position of the DYS

Mutation: when there is a change from father to son in the string of DNA on one of the alleles used for paternity testing. Example: on the Stewart Study, Noah's 3 sons descendants--two had 25 markers identical, 1 had one allele (marker) that was different. This was a naturally occurring event, but the question is, how often does that happen in a father son transmission.

Here is some information from a paper found on the link on the FTDNA page for understanding DNA (in quotes).  The paper is by Manfred Kayser and Antti Sajantila

"The common practice in paternity testing is that a difference at one or two out of 6-15 STR loci commonly analyzed is attributed to mutation rather than non-paternity, whereas differences at more than two loci are interpreted as non-biological paternity."

Transmission: DNA passed from father to son

In this study there were 4999 confirmed paternity father son transmissions. (fathers and sons DNA testings) for a baseline to determine how many mutations would naturally occur in that many births. The answer was:

"approximately up to eight of every thousand father/son pairs show a mutation and on average a Y-str mutation occurs in about three of every thousand father/son pairs.

Probability of Matches 12 vs 25 markers

Just as there are surnames with a high frequency of occurrence, such as Smith
and Jones, and surnames with a low frequency of occurrence, there are both
Haplotypes with a high frequency of occurrence, and Haplotypes with a low
frequency of occurrence. The 12 Marker result from the Y-chromosome 12 Marker
test is called a Haplotype.

When you compare a 12 Marker result to another 12 marker result of someone
with the SAME surname, and the results match 12/12, there is a 99%
probability that you are related. If you compare a 25 Marker result to
another 25 marker result for the SAME surname, and the results match 25/25,
then there is 99.9% confidence that the two individuals are related.

If you compare the 12 marker result to someone else who does not have the
same surname, but the scores match, you are most likely NOT related. When we
use the term related, we are talking about within the last 1000 years or 40

According to current theories, we are all related. The degree of relatedness
depends on the time frame, or generations between the participants and the
common ancestor.

If two 12 marker results match for two participants with the same surname,
and the genealogy research shows a common ancestor in 1835, the DNA test has
validated the research and proven that the two descendents are related. In
this example, you have two items of evidence to support that the individuals
tested are related. In addition, the research provided a precise time frame
for the common ancestor.

Without the genealogy research, and where 2 participants with the same
surname match on the 12 marker test, then the scientific answer to the degree
of relatedness is that 50% of the time the common ancestor would have
occurred within 14.5 generations, or within approximately 360 years. The
range of generations for the common ancestor extends to 76.9, or almost 2000
years. These long time frames exhibit the need to combine testing with
genealogical research.

The 25 marker test will more accurately determine the time frame of the
common ancestor. If 2 participants with the same surname match 25/25, then
the most likely time frame for the common ancestor is reduced to within 7
generations, or most likely within 175 years. When combining test results
with genealogical research, you can achieve a more precise time frame. If you
have a paper trail to a common ancestor, then the test result of a 25/25
match is confirming your research. If you have the paper trail, and you get a
15/25 match, you either have a problem with the research, an extra-marital
event, or an adoption in your family tree.

If you have a rare Haplotype, and you click the Find selection to search for
your matches, the results should all have your surname. When you have a rare
haplotype and you have a match with a different surname, this is most likely
the result of an adoption or extra marital event.
If you do match 12/12 with someone with a different surname, you may want to
compare your research to see if you can place any ancestors in the same
location at the same time, to begin an investigation of adoption or extra
marital event. Most likely you may also want to expand testing to include
other direct male descendents in both your trees for additional evidence to
determine when the event occurred. The preceding advice is for those with a
rare Haplotype.

If you have a high frequency, or common, Haplotype, the Find command will
provide you with many matches to many different surnames. Even though you
match, you are not considered related, in the sense that any relatedness was
probably to far in the past to have any genealogical value. Of course there
is the slim probability that one of the many matches of other surnames is the
result of adoption or extra marital event. The 25 marker results will help
identify this person.

It has not been scientifically proven why some Haplotypes are common and
others are rare. Many factors are believed to have influenced the situation
over time to produce common and rare Haplotypes. These factors would include
male birth rates, diseases and epidemics focused on localities, migration
patterns, mutation rates, etc. More scientific research is needed to have
more specific answers regarding why some Haplotypes are common and some are

It is important to consider whether your Haplotype is common or rare, to
effectively interpret results from 12 marker testing. If your Haplotype is
rare, and you have a 12/12 match with a different surname, it might be worth
your time to pursue this match, with comparing research and upgrading to the
25 marker test. If your Haplotype is common, and you match 17 participants
with 17 different surnames, you will probably not want to pursue these
matches. How many matches you get to different surnames indicates whether
your Haplotype is common or rare.

To more precisely define your degree of relatedness when you have the same
surname, and a 12/12 match, consider upgrading to a 25 marker test.

DNA testing is not meant to be a substitute for genealogical research. DNA
testing combined with your research and your surname is an effective tool to
resolve unanswered questions, determine relatedness, identify research
problems, and to prove or disprove theories or research.

With a 12 marker test, your degree of relatedness to another whom you match
12/12 involves both the surname and your genealogical research. If you don't
have a documented connection to a common ancestor, and you have a rare
surname and a rare Haplotype, you can assume that you are related with a
12/12 match. This conclusion is not reasonable for common Haplotypes.

For those with common Haplotypes, genealogical research and a 25 marker
upgrade is recommended. For those with a common Haplotype, a 10/12 and 11/12
match can be deceiving. They may or may not be related. In this case, we
recommend a 25 marker upgrade to determine the degree of relatedness.

In summary, the following chart applies combined with genealogical research:

Match Surname Haplotype Related?

12/12 same rare related
12/12 different rare possibly related-go to 25 markers
12/12 same common probably related-go to 25 markers
12/12 different common most likely not related

Reprinted with permission (Copyright 2002, Family Tree DNA)"Facts & Genes"


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