Genetic genealogy is a fast-growing field. The pioneering work in the field was carried out by late Professor Bryan Sykes of Oxford who, interestingly, himself happened to belong to one of the sub-branches of R1b-BY729. In his 2004 paper he studied correlation between the family name he bore and genetics within the area of West Yorkshire where the Sykes surname has the highest concentration [1]. From today’s perspective the genetic part of the study was fairly rudimentary as only 4 genetic markers were tested whereas recently 37-marker tests are considered a reasonable minimum to reliably dis/prove patrilineal relatedness. Despite the limitations Prof Sykes’ study clearly showed that about 40% of the tested individuals sharing the surname also shared the genetic imprint while the remaining 60% belonged to a number of different lines each represented only by a few individuals. Thus, a conclusion could be drawn that the Sykes surname originally represented male descendants of single man who flourished in Yorkshire. Only later did it spread to males of other genetic ancestry in a number of “small” events including adoptions, infidelity etc. which after several centuries created the pattern observed in the study.
Recently genetic genealogy jumped from scientific and university laboratories into the wide world and at the moment there are several companies offering the Y-chromosome ancestry test to anyone interested for (more and more) reasonable prices. This translates to rapidly growing databases of tested individuals which in turn allow to discern ever finer structure of the Y-chromosome haplogroups, their branches and sub-branches. These haplogroups and branches they form correspond to actual family lines of patrilineal descent.
While revealing these structures is purely a matter of testing more and more individuals and recent technology provides generally precise results in this respect, what remains an issue of great scientific as well as practical importance is extracting reliable time estimates from the mutations revealed by the testing. Let’s assume 3 living males take the Y-chromosome tests. These reveal that male1 and male2 share haplogroup h1. Further they show that they both share haplogroup h2 with male3 and the h2 belongs further back in time (higher up the genetic tree) since it has fewer mutations than h1. Structurally this translates to male1 and male2 having a common ancestor (A1) who in turn shared a common ancestor with male3 (A2). Thus, all the three living men are related but the mutual relation of male1 and male2 is closer (both genetically and temporally) than their relation to male3. What remains is the question WHEN did the common ancestors live?
Y-chromosome testing cannot provide an exact answer to this. Mutations which give rise to haplogroups are in principle random which means they can never serve as precise time-measuring device. The best we can hope for are estimates based on statistics. Since they occur relatively often compared to length of human lifespan, we can meaningfully calculate average interval for a mutation to occur. There is quite a lot of science to this but presently this interval is believed to be 85-100 years.
[1] Sykes, Bryan