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.
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 generations.
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 rare.
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"