Thursday, August 6, 2009

The Genetic Distances between Markers and the Affect it has on MRCA

Family Tree DNA has a great article published on its website about deciding how many markers to test in order to determine the time to the most recent common ancestor.

This is a screenshot of the MRCA statistics table on their website below:

I was trying to determine by how much the number of generations to the common ancestor increased as the number of genetic distances (differences between markers) increased. Based on the number posted above, I predicted the following:
It looks as if you have only 1 marker difference, the number of generations to your most common ancestor increases by 1/2 of the number of generations you started with.

For example, if you have a 37 marker test, and you are a 100% match, then there is a 95% probability that you shared a common ancestor within the last 7 generations. If you match 36/37 (1 marker difference), then the number of generations increases by 1/2 of the original number: so you started with 7 generations, take 1/2 of that which is 3.5 and add it to the original number = 10.5. So the number of generations to your common ancestor just increased from 7 to 10.5! If there is a 2 marker difference you would add another 3.5 to the 10.5 = 14!! These numbers are reflected in the table above and the calculations seem to apply to all tests.

I believe the standard for number of years per generation is about 45 years. So 14 generations would be about 630 years. Surnames only started about 600 years ago (~ 1400's), so anything beyond 600 years probably has little genealogical value.
I referenced Genealem's Genetic Genealogy Blog post article, "Why test 67 markers?" to come up with the standard of about 46 years / generation.
Genealem wrote: "25 marker match gives you a 95% probability of having a common ancestor within the last 600 yrs"
Combining the probability for a 25 marker match of 600 years and that of 13 generations as listed in the table above, the calcuated value is about 46 years / generation.
I originally wrote this article for family members who were trying to understand the genealogical value of DNA results. I am in no way an expert on DNA testing, but have developed a few tips and tricks along the way to better understand the results myself. This article reflects my findings.

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