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Calculating the time to your
most recent common ancestor

This page  computes probabilities for the time to the most recent common ancestor (abbreviated TMRCA) shared by two persons with a DNA match. It works as well for Taylors (or any surname) as it does for the McDonalds for whom it was intended. The program works by means of javascript; it will not work if you have blocked this type of content on your browser.

It was developed by Doug McDonald and originally published at http://dna-project.clan-donald-usa.org/tmrca.htm. A new version — much revised and including NextGen tests — has now been published at that location. 


You will need to enter three variables into the input boxes:

  1. Number of markers tested in common,
  2. Number of those markers that match
  3. Estimated average mutation rate. (Select from the guide he provides or use your own.)

Then check the "cumulative probability" box and click the "create graph" or "create list" button. The output will be generated below the buttons. To recalculate, change one or more input variables and click the button again.

TMRCA Calculator

This program calculates the probability that two people have a certain number of generations between them, based on the standard infinite alleles formula of Walsh. It calculates both the probability of being at an exact number of "transmission events" between a certain pair of people and the cumulative probability that the actual number of transmission events is less than a certain value. It can list both result types in a table, or graph either type. In either case the horizontal axis stops at the point where the cumulative probability reaches 97.5% or 10 generations, whichever is longer. Note that the number of transmission events is essentially twice the number of generations back to the MRCA (Most Recent Common Ancestor).

Suggested values for the average mutation rate per marker per generation are given below. These are estimates from all available sources by the author as of October 2006 and except for the FTDNA 12 set are still not very well determined. The SNP rates are particularly poorly known. More up to date estimates can sometimes be found in the Rootsweb genealogy-dna mailing list archives.

FTDNA Y12 STR:  0.0020   SMGF STR set (no 464): 0.0021   mtDNA complete genome3·10‑6
FTDNA Y25 STR:  0.0023 All 118 available Y STR: 0.0025 autosomal, X, and Y &SNPs 1·10‑8
FTDNA Y37 STR:  0.0033 mtDNA HVR:  2·10‑5  
FTDNA Y67 STR:  0.0028 mtDNA coding:  2·10‑6

This program was created by J. D. McDonald and is public domain.
See the October 2014 revision, which produces very different results.

   Number of markers          Number of markers that match

   Mutation Rate                  Cumulative Probability