Sandy, Thanks for the explanation. It does help, and your 134 year figure does seem more appropriate that 167 years for estimating the MRCA of my Sligo cousin and me. I did post the same question on another forum, and I received the following two answers from the man whom I gather came up with the 167 year figure: 1. "When counting GD between two men's to calculate a TMRCA requires the below formula which computes the average GD between two men. @ 67 markers - 7.5 transmissions (be) times 25 years per generation equals 167.5 years per mutation. (@ 30 = 225 yrs per mut) The TMRCA calculation would be: Using 67 markers for comparison between two men that have GD=20 between them: TMRCA=((mutations x be's) / 2 ) x YearsPerGen TMRCA=((20 x 7.5)/2)x25 = 1,875 years" 2. "The 167.5 years is an average I calculated using 0.2% (0.02) mutation rate which is 500 transmissions (birth events) per mutation. So 7.5be designed for 67 markers. And, I would use your known average number of years per generation instead of the default 25 or 30 years." I would suggest using McGee's YDNA utility with different mutation rate options were the TMRCA calculations use the average mutation rate for all the markers common between the pair of haplotypes being compared. McGee's 'Constant Rate' Default is 0.0024 which is about 416.6 transmission per mutation which is a tad bit faster at 67 marker 6.2be." I haven't tried using the McGee calculator (I don't understand MacDonald's charts), so that will be my next effort at the esoteric. One of the things that puzzles me though; when FTDNA (I use their calculator for GDs) says two men are 67/20, how can we assume that each is 10 from the MRCA? Perhaps one is 8 and the other 12, or doesn't it matter? Paul