TMRCA stands for “Time to Most Recent Common Ancestor.” It’s a measure of how long ago any two male individuals likely shared a common patrilineal ancestor. The TMRCA for any two men can be determined by testing and comparing Y-STR marker profiles. The strength and accuracy of the TMRCA prediction increases as more Y-STR markers are tested and compared.
The following examples show why testing more Y-STR markers can increase the precision of the TMRCA value:
12 Y-STR marker test:
A perfect match between 12 STR markers would predict your TMRCA to be approximately 14.4. This means that you and the person you compared with likely shared a common ancestor between 0 and 14.4 generations ago. This represents a very broad time frame and does not provide solid evidence that two individuals are from the same line.
20 Y-STR marker test:
With a perfect match for 20 STR markers, the TMRCA is narrowed down to 8.3. This means that these two people tested likely descended from the same line and shared a common ancestor anytime between 0 and 8.3 generations ago.
44 Y-STR marker test:
If two males match perfectly at 44 STR markers, the TMRCA becomes only 3.8. This means that these two people shared a common ancestor between 0 and 3.8 generations ago.
As you can see, the more STR markers you test and compare, the more precisely you can narrow down the time frame that you and another person shared a common paternal ancestor.