Introduction and Background Information: We all have DNA in every cell of our body. DNA is, in a basic chemical sense, what "life" really is - a self-replicating molecule. It has the instructions about how to make proteins and other substances that construct our very sophisticated and complex bodies. Over time the DNA molecules are altered periodically by mutations. A mutation can be caused by cosmic rays striking part of the molecule, exposure to chemicals in the environment, etc. Men have a Y chromosome. Women do not. A male gets their Y chromosome from their father. A mutation can be handed down from father to son. By looking at who has different mutations at different places on the DNA mo;ecule, populations and lineages can be separated out as to their relationships to one another and as to times when people and groups separated from a common ancestor. This metaphorical "branching" of the human "tree" is just a visual analogy for a chart.
Although, in its infancy as a genealogical research tool, DNA mutations or "markers" are being used to try to identify different populations, and eventually may resolve into identifying different families who share common DNA and share specific mutations or markers. This is already being routinely done with paternity testing and migration studies. The markers below have been identified by comparing the DNA of different individuals with the same surname. In addition to looking at the Y chromosome of males, mitochondrial DNA can similarly be used to trace the lineage of women. A third kind of "autosomal" DNA analysis can trace back even more of one's relatives, both the men and the women, but not nearly as far as the other two, which can potentially go back hundreds of thousands of years.
The study of DNA holds the promise of telling the story of how the human family tree has "branched" and also how families and individuals, the "leaves," are related to each other. The more that people participate in these studies, the more complete will be the information available to resolve the more specific questions about family lineages and history. At present the Y chromosome information is focused on the metaphorical "branches and leaves" of, Western Europe - The Irish - Irish Families.
The Callahan DNA Project at FTDNA (i.e., The Family Tree DNA website)
Brian G. Callahan Volunteer Administrator
"The majority of Irish men carry the R1b Y-DNA. This is true of most men in Western Europe. Most of the Callaghan men who have tested are from the R1b branch. From the R1b branch there are a number of leaves (SNPs) being studied that are particular to the Irish. We have a number of those leaves in our tree. I will point out the three major R1b clusters in our study:
R1b to L226: We have 14 Callaghan men who are presumed to be L226 or as some call it Type lll Irish. The L226 SNP (mutation ) is carried by many men with the O'Brien Surname (however L226 is not exclusive to the O'Briens). As I understand it the present day Chief of O'Briens of Thomond has tested and is L226. Some suggest that this is the Y DNA which was carried by Boru himself and his progeny. See http://www.irishtype3dna.org
R1b to DF21: We have 6 Callaghan men who are presumed to be DF21. These Callaghan men closely match a branch of McCarthy's. (Like the Callaghan's the McCarthy's have a number of branches off of R1b) Some have suggested that is the common connection between the 2 historic families. Time & more testing will tell.
R1b to M222: We have 17 Callaghan men who are M222 or Type 1 Irish. A 2007 Trinity College study correlated M222 with the famed & mythical Niall of the Nine Hostages. Some of our M222 Callaghan's are from the North so no surprise there. However some of the M222 Callaghan's are from Cork & Clare. See www.irishtimes.com/news/science/the-genetic-imprint-of-niall-of-the-nine-hostages-1.1771373 (good summary of Y-DNA Surname Studies) According to the National Geographic Genographic Project M222 “is the ancestral lineage of the Ui Neill and Connacht dynasties.”
J2
We also have some Callaghan men who are not on the R1b branch. 5 Callaghan men belong to the haplogroup J2 and look to be fairly closely related with one another. These men are a minority but their ancestors may have been in Ireland for a very long time. "
See also the reference to the Callahan DNA genetic markers at 42:25:
(Below) This has a good introduction to Genetic Genealogy.