Most Jamesons (regardless of any spelling variations) are not actually related to each other, at least not by blood lineage through just the paternal Jam?son line. Sure, you can be related, even by bloodlines, through other ways, like from your maternal side, as an example. We are interested here however, in establishing (defining) our Jameson family, based on an unbroken bloodline of just our (male pedigree bloodline) Jam?sons, going as far back as we can find them, if possible, all the way to that first person (of our bloodline) who called himself a Jam?son. Although this is very difficult to do by traditional methods (a paper trail), modern genetic testing can help us do this without any question or doubt (see here).
To actually be related to someone genetically (bloodline related), you need to be descended from a common ancestor. To actually be related to someone with with your same surname, you need to be descended from a common ancestor with the same bloodline (genetics) and with the same surname. The distinction here is important, because all of us are descended from ancestors older that the use of surnames and as such we need to understand that we have very old cousins who's descendants do not actually use the surname Jam?son at all. So to be clear, we are really just interested in those of our bloodline who use the surname Jam?son.
It is easy to assume everyone with the same (or similar) surname is somehow connected (if you go back far enough) to one original person. However, surnames weren't actually used until around the tenth or eleventh centuries, even later in some locations. So, you can not really go back "far enough" to that point where we were in fact all from the same original ancestor, if you are using surnames as a qualifier or category. This is especially not so with surname such as Jameson. Anyone can assume any surname, and sometimes, as is the case with the patronymic surname Jameson (son of James) the surname was often created spontaneously, even contemporaneously, without the slightest regards to any connection whatsoever. This does not mean that anyone you meet with the surname Jam?son is not related (we may not know enough about his/her history and genetic profile to determine that), but it does mean that anyone with the surname Jam?son may not be related.
Thus, there are several different unrelated family groups, each using the same Jam?son surname. Each is unique and each is descended from totally different original people(s). Our particular Jameson family is one of these unique Jameson families. This is not a case of hyperbole or hubris, this is a statement of fact and is scientifically provable. Furthermore, this is not unusual just to Jam?son families, most families using the same surname are not actually related to each other, particularly those with patronymic, occupational, or typographic surnames.
We do not know the original person who first used the surname Jameson in our particular and unique Jameson family, nor are we ever likely to with any kind of certainty. We suspect it was probably in the fourteenth or fifteenth century, maybe earlier, and he was probably a son, most likely the eldest son, of someone named James. As explained above and should be noted again, this would not be the earliest person with our specific genetic profile, and therefore not our earliest ancestor. This would also tell us that there are likely many more people out there with our exact same genetic profile, therefore technically family, but with entirely different surnames. This, by the genetic definition of family, presumes a hugely expanded group - almost beyond comprehension, with totally different surnames. This is an unexpected and entirely bewildering way to envision our particular family. Therefore, for our purposes it is much more acceptable and manageable to consider our family as just those of us with the exact same genetic profile, using the surname Jameson, or one of it's variable spellings.
Modern genetic testing can prove lineage, at least by the direct male descendancy. The YDNA chromosome is passed from father to (all) sons, identically (see our DNA section here). Therefore, incredible as it may seem, our family, at least all of the males with the Jameson surname directly descended from another male within the same family have that exact same (allowing very minor mutations) YDNA chromosome as each other for thousands of generations. One big genetic family. This is easily and affordably testable. Unfortunately, because the YDNA is not passed down from father to daughter, and no other genetic material is passed reliably for more then one generation, this line of provability is only available with male descendants.
So, our particular Jameson family can be defined as those men with the provable YDNA profile and any woman who can be shown to be related to any male with that YDNA profile by other acceptable means.
Criticism of this "genetic" genealogy is that it is heartless and cold and does not embrace the culture and nuance of actual families. However true this might be, it can be said it is no less true or fundamentally any different then more traditional methods of genealogical information gathering, nor does it exclude anyone from gathering any other data or materials that make up a proper genealogy or family history. What it does do is convincingly prove inclusion, and at the same time it is a whole lot more accurate and definitive. Furthermore, it is basically similar methodology, just different - science.
As it turns out genetic testing now shows there are already several totally different Jameson Family groups and these in at least five altogether different Halpogroups. This at a time when we are still relatively new to genetic testing with perhaps less then 5% of those with the surname Jameson have been tested. It is in fact, safe to say there are probably more people with the surname Jam?son who are not related to each other then there are those who are related to each other.
Our particular Jameson family is indeed a unique, distinct, independent, stand alone, Jameson family. We have our own beginnings and our own history. There are many families with the surname Jameson, most however, are not related. We are just one.
Our Particular Jameson Family - Defined and Explained