Most Recent Common Ancestor Analysis (MRCA)
MRCA and MLE are terms that are used often in discussions of genetic genealogy. MRCA stands for Most Recent Common Ancestor and it refers to a statistical calculation that determines the likely generation in which two individuals share a common ancestor. 
The MRCA is often reported with a 95% Confidence Interval. The MRCA number itself is the generation in which it is 50% likely that two individuals are related. The 95% Confidence Interval then gives you a range of generation values that encompass 95% of all possibilities. For example, there is a 95% likelihood that two individuals sharing 25 of 26 alleles will have an MRCA within 11.9 generations expanding between 1.7 and 39.5 generations. The MLE or Maximum Likelihood Estimate, in this example is the most likely generation in which the two people are related. In this case it is 7 generations. (See illustration to left.)
The following table displays the MRCA calculation, the 95% Confidence Interval, and the MLE for the Relative Genetics 18, 26, and 43 marker tests. All numbers represent the number of generations back to the common ancestor. Notice that the MLE for a perfect match is always zero. This is because statistically, the most likely scenario for a perfect genetic match is that the two genetic profiles have come from the same individual. More information about these categories can be found in the Guide to Understanding Your Matches section.
It is important to remember that when looking at the MRCA or MLE information these numbers are to be used as a guide. Given substantial genealogical evidence to support a relationship sooner or later than the MRCA or MLE, is probably accurate as long as your estimation fits within the confidence interval. This is why the marriage of genetics and genealogy is integral to the success of any genetic genealogy endeavor. It helps to think of the MRCA and MLE as functions of the GPS- Genealogical Positioning System- helping you and your family verify genealogy or start you out on the path to finding a previously elusive genealogical connection.
| Match Resolution | MRCA | 95% Confidence Interval | MLE | |
|---|---|---|---|---|
| Lower Limit | Upper Limit | |||
| 18/18 | 7 | 1 | 27 | 0 |
| 17/18 | 18 | 3 | 56 | 10 |
| 16/18 | 29 | 7 | 77 | 22 |
| 26/26 | 5 | 1 | 18 | 0 |
| 25/26 | 13 | 2 | 38 | 7 |
| 24/26 | 20 | 5 | 52 | 14 |
| 23/26 | 28 | 9 | 65 | 22 |
| 43/43 | 3 | 1 | 10 | 0 |
| 42/43 | 8 | 1 | 21 | 4 |
| 41/43 | 12 | 4 | 31 | 9 |
| 40/43 | 17 | 6 | 39 | 13 |
| 39/43 | 21 | 8 | 46 | 17 |
| 38/43 | 26 | 11 | 53 | 22 |
