MRCA stands for “Most Recent Common Ancestor”.  When comparing two individuals, the MRCA is the most recent ancestor from which the two individuals descended.   

TMRCA stands for “Time to Most Recent Common Ancestor”.  It’s a measure of how long ago two individuals likely shared a common ancestor.

Determining TMRCA through DNA testing:  On the paternal line, the TMRCA for two individuals can be predicted by testing markers on the Y-DNA called STR markers.  The more STR markers that are tested and compared, the more the test can narrow down the TMRCA value.

Example:

12 marker test:  If you and someone else test 12 STR markers, and matched each other perfectly at all 12 markers, your TMRCA is approximately 14.4. This means that you and the other individual likely shared a common ancestor between 0 to 14.4 generations ago. Now that’s a very broad time frame and does not provide solid evidence that two individuals are from the same line.

20 marker test:  If you and someone test more markers, such as 20 STR markers, and matched each other perfectly at all 20 markers, your TMRCA is narrowed down to 8.3. This means that you and the other individual likely descended from the same line and that you shared a common ancestor anytime between 0 to 8.3 generations ago.

44 marker test:  If you test 44 markers and match perfectly at all 44, your MRCA becomes only 3.8. This means that you and the other person shared a common ancestor between 0 and 3.8 generations ago.

As you can see, the more STR markers that are compared, the more informative the results.