Lessons Learned: Estimating Distance to a Common Ancestor

Understanding DNA tests, genetic markers, and their application as a genealogy tool requires advanced degrees across several disciplines--Genetics, Calculus, Statistics, Social History, Immigration History. However, we all can understand the application if we reduce the study down to the basics so we can read along with what the experts write.

By no means am I an expert in the field. However, I spent some time getting up to speed after I submitted my sample and came up with the matches to two living cousins. Having the results which indicated somebody somewhere in the world matched me was nice. The question was, “How can I use this new information to further my search?”

Note, geneticists not genealogists came up with these descriptions and names.

1. Genome: our genetic sequence
2. Mutation: changes in the genome
3. yDNA: the genetic sequence passed down from father to son
4. mtDNA: the genetic sequence passed down from mother to daughter
5. atDNA: all the genetic combinations passed down from all the other ancestors.
6. Loci: different locations along the genetic sequence; commonly referred to as markers
7. STR: Short Tandem Repeats are markers where there is a high probability for a mutation; these markers are used by genetics tests to determine relationships
8. SNP: Single-Nucleotide Polymorphisms are markers where there is an extremely low probability of mutation; these markers are used to determine Haplogroups and their inherent subclades
9. Allele: genetic information found at a particular marker; the number of repetitions of a genetic code at one marker; commonly referred to as knots
10. Haplotype: your personal genetic information
11. Haplogroup: a name given to a group of similar genetic sequences. A new letter marks a major mutation which branches one group off from another. I am Haplogroup R. My ancestors went from B to F to K to P to R.
12. Subclade: a minor mutation within the Haplogroup. We, the Proto-Celts from the Atlantic Fringe, are R1b aka  Cro-Magnon Man ; R1a are in Central Europe and on the Asian Steppes; R2 went to Pakistan and India.
13. MRCA: Most Recent Common Ancestor; statistically how far back you have to go to find a connection to another person who matches you through a DNA test.
14. TMRCA: Time to Most Recent Common Ancestor expressed in years or generations.
15: Donor: Me.

Basic Concepts: 

1. As the human genome ages, it changes. At one time every Homo Sapien Sapien (modern people) had the same genome. The original sequence has been designated Haplogroup A (no subclades). They lived in Africa and never left. And reportedly, are non-extant.
2. As time went by, the genome changed in some people; especially the folks who migrated out of Africa into the Steppes and then across to Europe. Note, Haplogroup R is the farthest down the Haplogroup Chart from A.
3. Evidence for the change is found by counting the “knots” on the DNA sequence at specific locations (loci/markers).
4. How is this done? Per the FTDNA webpage, scientist #1 does a count. Then scientist #2 does a different count. And scientist #3 does a comparison. If they don’t match; they start over.
5. DNA tests which have a mutation at a specific SNP are grouped into a Haplogroup. To be R1b, you must have a mutation at SNP M343.
6. As part of your test results, there will be an estimated Haplogroup which is based on the initial STR markers found. My Haplogroup is estimated to be R1b1a2. And, the Haplogroup is posted in red to indicate is is an estimate.
7. Some people want to find the definitive answer. Another cousin paid to have the Deep SNP test. FTDNA measured all known SNP mutations and found one at P312. That means that we are R1b1a2a1a1b. And, my cousin's Haplogoup is displayed in green.

There is a chart of my family's genetic history at my DNA page. The information is copied from the National Geographic Genographic Project where I am a contributor. As you go down the DNA Chart, you will find that R1b1 is the group who settled the British Isles at the end of the last Ice Age. Effectively, my family branches from the Proto-Bretans of the Neolithic Period. Our people built Stonehenge. Historical information has yet to be associated with the additional markers as you move further down the chart

Having obtained your DNA test results, the question is whether or not there will be a match and what to do with the information when there is.

Analyzing Information from the Match:

1. Using the 37 marker DNA test as the standard, the markers in the first group (01-12) mutate at the slowest rate (.002), the middle group (13-25) mutates at a faster rate (.003), the final group (26-37) mutate at the fastest rate (.004 - .005). Note, these are approximate averages across the entire group and not a specific rate per marker. These mutation rates are based on actual family studies where the living donor did not match the proven ancestor. Imagine someone asking you to do a DNA test and then getting permission to dig up your famous ancestor to test the corpse. And, there are three famous studies where that’s exactly what they did.

Note: The FTDNA Calculator effectively does the remaining steps for you and provides information for multiple-step mutations.

2. For a more precise one-step mutation, go to the MRCA Calculator. Input the number of markers. Input the mutation rate based on the group where the mutation is found. Calculate for "Generations to MRCA." Your best answer is the first generation which is equal to or exceeds 50%. The answer indicates the number of generations up the family tree to the ancestor you share with your DNA match.

3. Take out your genealogy paper record. Go up the family tree the associated number of generations. The person on the tree directly above is, statistically, your Most Recent Common Ancestor.

And, finding that person is one of the many purposes for this webpage.

Charles E. Lewis
Davenport IA 52802


This site is provided for reference only. Except where specifically cited, information contained is conjecture and should not be considered as fact.
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