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Blair Genealogy


DNA 101: Y-DNA

DNA 102: Interpreting Results

DNA 103: Grouping Participants

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Oldest Ancestors


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DNA 102: Interpreting DNA Test Results

Almost the first question a Project Administrator gets from a participant is "I've just got my test results, what do they mean?" Most, who have run a DNA Project for any length of time, will probably agree that interpreting DNA test result is not always easy or straight forward, and usually gets more difficult as the size of a project grows.


  • What DNA Testing Can Not Do

  • What DNA Testing Can Do

  • Exact Science Combined with Probability


DNA Testing


to be used along with
Conventional Genealogical Research

DNA testing has little value on it�s own, but when used with other conventional methods of tracing your ancestry, DNA becomes a valuable tool.


It�s important for both Project Administrator and participants to understand the limitations of DNA testing. There are a number of things that DNA testing can NOT do.

DNA Testing Can Not

  • Tell you who your ancestors are. A person�s test results on their own have virtually no value. You can�t plug your DNA results into some magic formula and find out who your ancestors were. You need to compare your results with the test results of others to determine if you may somehow be connected.

  • Tell two participants with matching (or near matching) test results who their common ancestor is. Even with an exact match the results won�t tell you who your common ancestor is.

  • Tell two participants with matching (or near matching) test results exactly how far back their common ancestor existed.

  • Prove a suspected connection between two participants with a paper trail.


So what CAN DNA testing do?

DNA Testing Can

  • Indicate that you and another participant share a common ancestor.
    If two participants closely match on their test results this is an indication that they share a common ancestor. Just how strong this connection is depends on the strength of the DNA match which I�ll discuss later.

  • Give you a ROUGH idea of how far back your common ancestor lived .
    This also depends on the strength of the DNA match plus the paper trails.

  • Provide evidence that suspected lines are connected.
    While DNA testing can not prove that suspected lines are connected, it can provide evidence to support that premise. How strong this evidence is depends on the paper trails and the strength of the DNA match.

  • Provide proof that two individuals or suspected lines are NOT connected .
    No matter how good the paper trail may be,  if there are too many DNA mutations, they can not be related.


Genetic Genealogy is somewhat of a dichotomy. On one hand we use the extremely accurate and precise science of DNA testing to get our test results and on the other hand we then take the test results and apply approximated mutation rates and probability to interpret the results.

Exact Science Combined with Probability

  • The actual DNA testing procedure is an extremely accurate and precise science.

  • Using the test result involves applying probability and inexact estimates of mutation rates.

  • Must use statistics and probability to estimate the Time to the Most Recent Common Ancestor (TMRCA)

  • The actual calculations of TMRCA are mathematically complex and depend on knowing the number of mutations and the rate of mutation.

One of the primary purposes of DNA testing is to determine if two participants share a common ancestor and hopefully, by using conventional research, determine who that common ancestor is.

The most common method to do this is to calculate the estimated Time to Most Recent Common Ancestor (TMRCA). In other words, estimate how many generations you have to go back to find the common ancestor of two participants.

To do this you have to use statistics and probabilities.

The actual calculations of the TMRCA are far too complex to discuss here, but they depend on knowing the number of mutations and the rate of mutation.

Number of Mutations

There are two common ways to count the number of mutations.

  • The first method, called the Infinite Allele Model, counts any change in a marker as a single mutation. Each marker is scored as either a match or a non-match. If a marker does not match it is assumed to be a single mutation.

  • The second method considers the actual difference between the values of markers that do not match. These differences are then added to give the genetic distance.

  • Example:

In the example shown, XXX mismatches YYY on 3 markers, but two of the markers have a difference in their value of 2 giving a genetic distance of 5.


The second factor in computing the Time to the Most Recent Common Ancestor is the mutation rate of the markers.

Mutation Rate

The mutation rate has a marked effect on the calculation of TMRCA. 

  • Doubling the average mutation rate effectively cuts the TMRCA in half. For example, if you have a match on 36 of 37 marker:

    • an average mutation rate of 0.002 produces a 50% chance of sharing a common ancestor within 12 generations.

    • an average mutation rate of 0.004 produces a 50% chance of sharing a common ancestor within 6 generations.

  • One of the most important things to remember is that mutations occur at random

  • Can�t be predicted.
    There�s no way to predict which marker will mutate or when a marker will mutate.

  • The mutation rates used to calculate the Time to Most Recent Common Ancestor are estimated based on past observations.

Despite the critical nature of mutation rates in calculating the TMRCA there is still no real consensus on either individual marker or average mutation rates. Various studies and companies have attempted to determine the mutation rates of individual markers as well as average mutation rates for groups of markers, but the rates seem to vary from study to study and from company to company. Even with all the DNA testing that�s been done, given the random nature of mutations, there is still not enough data to reach agreement on mutation rates.

There are links to several sites with differing mutation rates at the bottom of this page.



FamityTree DNA has developed a program, called FTDNATip, that automatically calculates the TMRCA between two participants in a surname project. It uses individual marker mutation rates that FTDNA has developed. FTDNATip has been criticized by many for using mutation rates that are too high, thus producing results that are overly optimistic.

Shown below is a reproduction of an actual FTDNATip report of two participants in the Blair DNA Project.


 There are two things you should notice:

  • First, the two participants mismatch on 2 markers but they have a genetic distance of 3, meaning that they mismatch on one of their marker by two.

  • Second, the probabilities are stated as WITHIN a certain number of generations, i.e. a 68.67% probability of sharing a common ancestor WITHIN 8 generations.


Time to the Most Recent Common Ancestor

TMRCA is a very broad estimate based on uncertain mutation rates. The probabilities will vary depending on the mutation rates used.

It�s imperative that you realize that the probabilities are WITHIN x number of generations. If the results state 89% within 12 generations it means there�s an 89% probability that your common ancestor existed sometime between generation 1 and generation 12. It does not mean there is an 89% probability your common ancestor was 12 generations ago.

The TMRCA results are based solely on the number of mutations and the mutation rates. It doesn�t know your surname or the surname of the participant you�re comparing your results to.

The TMRCA results don�t not know anything about your genealogies. It does NOT know that you paper trail goes back 3, 4, or 10 generations without a common ancestor.

All it knows is that you and your comparison have X mutations and you are using a mutation rate of Y.


TMRCA Alternative

For those that don�t like dealing with statistics, FTDNA provides �Guides for Interpreting Genetic Distance within Surname Projects� which are descriptive rather than mathematical. There are guides for 12, 25, 37, and 67 marker tests.

For example on 37 Markers you find:

  • Distance: 0 - Very Tightly Related

  • Distance: 1 - Tightly Related

  • Distance: 2 or 3 - Related

  • Distance: 4 - Probably Related

  • Distance: 5 - Only Possibly Related

  • Distance: 6 or more - Not Related

Links to these guides are provided at the bottom of this page.

Putting it All Together

DNA testing can be a valuable tool in
genealogical research when combined
with conventional research

Test results can be used to support a
suspected connection between two families
or disprove a connection

Although it is impossible to pinpoint a common
ancestor from the test results alone, with a
proper paper trail you may be able to do so.


DNA 101: Y-Chromosome Testing

DNA 103: Grouping Participants


DNA References for
Interpreting DNA Test Results

Sorenson Molecular Genealogy Foundation (SMGF) - Y-Chromosome Database

Marker Mutation Rates Marker & Mutation Comparison -

Leo Little - Mutation Rate Effects -

Wikipedia - List of DYS markers -

Marker-to-DYS Conversion Chart with Mutation Rates -

TMRCA Calculators

Time to Most Recent Common Ancestry Calculator by Bruce Walsh -

Clan Donald USA TMRCA Calculator -

TMRCA Calculator -

Moses Walker TMRCA Calculator -

FTDNA Interpreting Genetic Distance within Surname Projects

12 Markers -

25 Markers -

37 Markers -

67 Markers - h

This WebPage was last updated 04/05/2015

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