Applying the Generations to Most Recent Common Ancestor Calculator

One of the features of FTDNA is a calculation of the probable distance to the Most Recent Common Ancestor. A variable that must be entered into this calculator is the known number of generations without a common ancestor. In this instance, I know for certain that my father (Harry), grandfather (Mathew), and great-grandfather (Mark) are not common with Tommie Smith, so I entered the number 3.

With that information FTDNA calculates a 52.72% probability that we had a common ancestor within the last four generations. The probability increases to 91.94% for the last 8 generations; and 98.83% within the last 12 generations. Based upon our individual research Tommie Smith and I believe there is a strong possible, but unproven, connection four generations ago or at least by 5 generations.

My next closest matches at the 67-marker level are Norman Elijah Reeves and Bruce Reeves with a genetic distance of three. Using the FTDNA calculator it is estimated that the likelihood of a common ancestor within the last four generations is 18.26%, and 64.50% in the last 8 generations. However, if I change the number of known generations without a common ancestor from 3 to 4 then the probability of a match in four generations falls to 11.50%, and 61.57% in 8 generations.

Similarly, I have a genetic distance of 4 with two others at the 67-marker level: Robert Wynne Kauppila and Antone Augustus Reeves. Using three as the "known generations" variable then we have a 7.17% probability of a common ancestor within 4 generations, and 42.29% probability within 8 generations. If I change the variable to 4 known generations without a common ancestor then the percentages fall to 4.82% in four generations and 40.83% in 8 generations.

Next, I created a chart listing the most recent ancestors (best proven) for four of the individuals with whom I most closely matched in the 67-marker test (one person who matched does not have a family line posted).

Researcher	Richard	Tommie	Rebecca	Bruce	Beulah
Tested Person		Stephen	Norman		Antone
Distance	0 (self)	-1	-3	-3	-4
Line	34977	28839	122068	113046	31962
		Billy (1931)	Elijah (1930)	Joe (1934)
	Harry (1914)	Thomas (1896)	Janes (1889)	(Thomas (1907)	Antone (1912)
	Mathew (1868)	Elijah (1885)	Henry (1857)	William (1853)	Antone Sr (1877)
	Mark (1834)	James M (1823)	James W (1825)	Joseph (1816)	John (1813/16)
	Leroy (1803)	Thompson (1799)	Morrison (?) (1795)	John (1786)	Joel (1775/80)
	Malachi (1779) (possible)		John R (1776)
				Jeremiah (1738)	Jeremiah (1738)

[Note: Some of the cells were shifted down in an attempt to line up generations chronologically.]

Conclusion

Tommie and I have long suspected that Leroy (1803) and Thompson (1799) might be brothers. That means our common ancestor could be their father, a supposition that is consistent with the MRCA calculator described above that provides a 52.72% probability that we have a common ancestor within the last four generations and a 91.94% probability that the common ancestor lived within the past eight generations.

The likelihood of a common ancestor with Norman and Bruce within the past four generations is only 18.26%. This might suggest that Leroy, Thompson, Morrison and John are not brothers, but perhaps Morrison and John are cousins to Leroy and Thompson. The probability of a common ancestor within eight generations for these four ancestors is approximately 40 percent.

Remember, this is all speculation, but it might provide some pointers to future research.