# 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.

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.

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.