BLAIR DNA Project
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.
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
So what CAN DNA testing do?
DNA Testing Can
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
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.
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.
The mutation rate has a marked effect on the calculation of TMRCA.
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:
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.
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:
Links to these guides are provided at the bottom of this page.
DNA References for
Sorenson Molecular Genealogy Foundation (SMGF) - Y-Chromosome Database http://www.smgf.org/pages/ydatabase.jspx
Marker Mutation Rates
WorldFamilies.net Marker & Mutation Comparison - http://www.worldfamilies.net/marker
Leo Little - Mutation Rate Effects - http://freepages.genealogy.rootsweb.ancestry.com/~geneticgenealogy/ratestuff.htm
Wikipedia - List of DYS markers - http://en.wikipedia.org/wiki/List_of_DYS_markers
Marker-to-DYS Conversion Chart with Mutation Rates - http://micbarnette.bravepages.com/dys_conversion_chart.html
Time to Most Recent Common Ancestry Calculator by Bruce Walsh - http://nitro.biosci.arizona.edu/ftdna/TMRCA.html
Clan Donald USA TMRCA Calculator - http://dna-project.clan-donald-usa.org/tmrca.htm
TMRCA Calculator - http://www.dnacalculator.org/tmrcaCalculator.php
Moses Walker TMRCA Calculator - http://www.moseswalker.com/mrca/calculator.asp?q=2
FTDNA Interpreting Genetic Distance within Surname Projects
This WebPage was last updated 04/05/2015
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