Michael Cooley's Genetic Genealogy Blog GEN • GEN
3 May 2016

Y-SNP Discovery, Part II

Go to Part I

Many of you, if not most, have limited interest in this. Some of you were persuaded by relatives to give up a DNA sample and, in many instances, your test was paid for. Because my interests have varied considerably throughout my life I have learned that the appreciation of a subject comes along with the understanding of it. It's kind of another version of "if you build it, they will come." Hence, these postings.

The FTDNA bulk email interface is problematic for me, so I've taken to posting these at my ancestraldata.com website. Only yesterday I decided to start a blog. Eventually, I'll begin posting there instead.


I tried another tact in my last email in describing Y-DNA mutations or SNPs (Single Nucleotide Polymorphisms) and their part in uncovering our ancestry. To summarize, just as we can determine how closely related two people are by counting the number of ancestors they have in common, we can use the same method with the father-line by counting the number of Y-DNA mutations that had come into the lineage. (Remember, every new mutation occurs at the conception of a man and all accumulated mutations pass to his sons—just as all accumulated ancestors do!) In the first case, the siblings share ancestors A, B and D but only A (their great-great grandparent) with the second cousin.

By looking once again at the CF01 SNP tree, we find that I (A7497) share six of the represented SNPs with A7411 but only four with the Cochrans. We know, then, something of the degree of relationship between these families.

But what else do we know? YP4248 is estimated to be about 800 years old. To date, everyone who has tested positive for that SNP has origins in the British Isles, possibly Scotland. However, YP4252 has its origins in Norway. This could place the lineage smack dab in the middle of the Viking invasions.

But these SNPs are not tested for on an individual basis. The Big Y examines ten million positions on the Y chromosome. The vast majority of them match with other testers. Those that do not match are dubbed "novel" SNPs, but they are novel only until the time matches are found for them. For example, David Cooley (the former president of the Cooley Family Association of America) has twenty-three novel SNPs. But many of them will be matched once another descendant of Benjamin Cooley of Massachusetts tests. Similarly, once the CF01 Hackett tested, he matched only to other YP609 descendants and had eighteen novel SNPs left over. But those shrunk to five after other testers entered the game.

So what happens is this. Virtually every SNP is discovered in blocks of SNPs like David's block of twenty-three. The blocks are reduced once new matches occur. To illustrate the point, these five slides (they advance every four seconds) show how the Hackett SNPs (in green) have been parsed out as new matches are found. (The total count of the "green" SNPs never changes.) The following includes the Hackett tester, two Cooleys, and later a Mann and a Berge (see the previous diagram for a fixed version):

Many hundreds, if not thousands, of SNP blocks can be found on the worldwide Y-SNP tree. Theoretically, all of them can be broken down into individual SNPs and sorted into a timeline. But any lineage, on the other hand, that would have only the YP4492 SNP found in the top Cooley block, for example, could have become extinct. In that event, it would remain in the YP4491 block, its fate pondered for all eternity. And generally, the older the block, the more SNPs it will possess and the less likely a sufficient number of testers will be found in order to fully sort it out. But there's a lot of progress yet to be made.

Once the second Benjamin Big Y is completed (and we do have a volunteer) we'll have similar results, and just as we know that my John Cooley of Stokes County, North Carolina was born with the YP4491 block (and all its ancestral SNPs), we'll have a similar breakdown for Benjamin. This will also be true once the test results arrive and are analyzed for the second tester in the John A Cooley (1753-1794) line. And once some of this is sorted out, I'll be able to use something other than my own family as an example!