Michael Cooley's Genetic Genealogy Blog
23 July 2020

Update to the Pettit/Mellowes SNP Tree

Y-SNP analysis can be as easy as ABC 123. The results can draw a beeline directly from the tester and straight up the male line to his earliest paternal ancestor (or, at least, as far as he can identify his Earliest Known Ancestor, or EKA). For example, I have the Y-DNA marker R1a-YP4491. It's been proven several times over that my 6th great-grandfather, John Cooley (c1737-1811), had the same marker! That this could be done has been a wake-up call to genealogists. In fact, nothing can be more straight-forward. Y-DNA analysis works because the Y chromosome clones itself from one generation to the next. It's not cluttered with that messy autosomal stuff. (Well, that's half true, but we have no need to go there — ever, really.)

Y-DNA cloning is a bit like the repeated copying of one Xerox copy to the next. We can often recognize a photocopy of a copy because resolution is lost with each generation. The edges becomes increasingly fuzzy, and the blacks begin to break up. But because it's generational, a person with reasonable analytical skills can take a shuffled pile of photocopies and arrange them in order to create a timeline of sorts. It boils down to simple comparative analysis, a skill that college students are (or should be) taught.

DNA, however, doesn't lose resolution. It shifts. In a way, it gains resolution with each mutation that creeps into the lineage, perhaps once or twice every few generations. And like our pile of photocopies, we can look at the results of several testers, compare them, and determine the order in which the mutations came into the lineages.

The value of SNPs

The markers of interest are those arising from a single-letter mutation from one base to another of the four genetic letters (a G to an A, for example). These are called Single Nucleotide Polymorphisms, or SNPs. Those SNPs appearing in stable regions of the Y — unlike those in the centromere or fickle STR regions — are highly valued. Like the photocopies of the earlier example, they can be arranged in order. Some Y-SNPs are shown to be more than 300,000 years old while others might have emerged only two or three hundred years ago. Indeed, there are living males in whom unique SNPs first manifested. Theoretically, they're so unique that they're not present in any other living person besides the carriers' own sons and grandsons. As soon as these markers see the light of day, they're archived into every cell of their host, right alongside all other SNPs ever born into the lineage. Their order, however, can be determined only by comparing results across multiple testers.

For this reason, I often proclaim that "SNPs are people too." It's not literally true, of course. But the phrase does reference a critical fact — that Y chromosomal SNP mutations first appear at the births of specific men at specific places at specific times. I refer to SNPs that have been fully identified as anchor SNPs. Theoretically, some form of birth certification can be issued for them, which might include their paternal lineages over tens of thousands of years.

Of course, the genealogy always needs to be done, to the extent that it can be. But Y-SNP sequencing and analysis has allowed for the sorting of the nebulous Pettit-Mellowes cloud to something having form — from,


to
R1b-BY200368 SNP Tree

Among the Big Y testers, only the Pettit/Heaths have the lineage worked out to Thomas Pettit (1609-c1668) and his wife Christian Mellowes. Previously, that there might have been a connection between these lineages was only speculative. That a connection exists is now a genetic fact, although we'll need to return to the record to see just how. Still, research can proceed with a level of improved confidence, even of certainty, that these families are descended either directly from Thomas and Christian or from closely-related male relatives. In that sense, it's a done deal.

The new tester, kit #914894, is a descendant of Elias Pettet of Westmoreland County, Pennsylvania and a cousin of mine. Back in the days of The Pettit Correspondent I resisted lumping our Pettets with Thomas and Christian, something that virtually all American Pettits wished to do. I took several opportunities in those publications to stress that the record clearly shows that there were plenty of other immigrant candidates. Naturally, I heeded my own advice and shoved aside the notion of a Thomas descent. Now, the genetic record is clear; if not of specific descent, of familial descent.

The takeaways

Now that there are six Big Y testers, it's time to pause and reflect on the general impressions presented by the current data. First, we now have three genetically-defined branches in the Pettit-Mellowes lineage. Further testing will show additional branching. Next, Elias Pettet appears to have been more closely related to Pettit/Clayton than to the others. Of course, the pair's Most Recent Common Ancestor (MRCA) might have been Thomas himself, but it's possible the MRCA lived several generations later. In any case, this is of great benefit to my personal genealogical research. I'll be looking more closely at William Pettit (c1779-) and Rebecca Clayton.

The biggest takeaway is that the SNP tree is quite top heavy. If we consider that Y-SNPs might have come into the lineage, on average, about once every fifty years or so, the 22 SNPs now present in the BY200368 haplogroup would have emerged over about eleven hundred years. In contrast, the remainder of the tree is sparse in SNPs. If we tally up the few SNPs that descended from BY200368 and divide by the number of testers, we get an average of barely a two-SNP descent to the testers. That would put the MRCA as living only about a hundred years before present (defined as 1950), perhaps the mid-1800s. These are spindly legs to support such a large body. There's a bigger story waiting to be gleaned.

No matter how we fudge the numbers, we can't arrive to a post-BY200368 birth for Thomas in 1609. I suggest that those SNPs that made up his Y-DNA birth print are buried somewhere in that large block of SNPs, that he didn't have the others, and that other Pettits of this clan do not have all of them. And it's possible we might whittle down the large BY200368 block. But I'll post about that later on the project's discussion group at Facebook. (See below.)

Every Big Y test provides more information. They clarify relationships to one degree or another, and they suggest what paths to follow for both genealogical and genetic research. It's clear that more testing will open that top-heavy batch of SNPs and provide considerably more articulation to the tree. The trick is in discovering which Pettits will provide that finer detail.

The future

Apart from shattering the BY200368 haplogroup in a way that is not now clearly defined, specific advanced testing will provide specific detail that needs to be (and can be) illuminated. So far, the only known (tested) descendants of Thomas come from Pettit/Heath subclade, presently defined as BY198412. Because the testers are father and son, we don't know at what generation those SNPs emerged — perhaps with the father, his father, or some generations further back, which is more likely. Another descendant of another son of either Nathaniel Pettit, or further down the tree, will help isolate at which generation the SNP was born. Since the lineage's genealogy is well known and includes numerous living descendants, it's well within reason to suggest that we'll discover an anchor SNP as defined above.

On the same note, we have two testers for the Pettit/Gallagher line. The three mutual SNPs triangulate back to Samuel Pettit Jr (1790-1874) and Sarah Gallagher. In other words, we have a verifies Y-DNA print for Samuel. Anyone testing positive for them will be either a Pettit/Gallagher descendant or a descended from a close relative of Samuel's. But did Samuel Sr have all three? A descendant of his brother, Isaac Pettit (1809-1870), has that information hidden within each cell of his body. I highly recommend advanced testing for that lineage.

The Samuel and Sarah lineage has even more potential than that. Each of the three SNPs were likely born in three different individuals. Some of you will have one, two, or all three of them. Once those individuals are revealed, we will have drawn a genetic link between those families and have made an important step forward. We will have also closed much of the gap between Samuel Sr and Thomas. And that would be good news and highly demonstrative of Y-DNA's power. There's real potential for Pettit/Gallagher lineage.

Please participate

I want to make it clear to everyone that I'm not an agent of FTDNA or any other for-profit entity. Profit is not my motivation. Knowledge moves me. Still, because I am a descendant of this clan, I have a personal genealogical interest in seeing it grow. If fact, my excitement escalated with every sentence I've written here. My experience is that this kind of enthusiasm can be infectious. I hope I have duly infected some of you.


The discussion component for the Pettit-Mellowes Project is at Facebook,

For now, I have the raw results for the project at the Cooley DNA Project at FTDNA. The Pettit-Mellowes section is located at the very bottom of the page. (Some results do not appear on this public page. I would like to ask the testers to change the permissions in your account so that they do. It makes for good advertising, the modern equivalent of the old genealogical queries sections of genealogical magazines and newsletters.)

Here's the home page for the Project. It will evolve in the coming months and acquire a new URL.

And I've written twice before about this project.

As always, I'm happy to answer any and all questions about this.