Our y-DNA Mutation Rates

DNA strand


Continued from The O'Rourke y-DNA -- an update, November 2015.

Reading Kerry Scott's article, Why Your DNA Test Results Are Wrong, got me to put down in writing what I had been thinking about for a while. When you put my family's paternal Hamilton y-DNA results and our maternal O'Rourke y-DNA results side by side, something doesn't really make much sense.

By Googling "y-dna average mutation rate" I found a few articles, which I don't pretend to have read thoroughly, let alone understand. However after skimming them I think our family's results seem to bear out the conclusions these people reached.

The articles are:

An Overview and Discussion of Various DNA Mutation Rates and DNA Haplotype Mutation Rates. Do the YSTR Haplotypes in some Y Chromosome Male Lines Mutate Faster Than in Other Male Lines?, By: Charles F. Kerchner, Jr., P.E. (Retired), Written: 7 Jan 2005, Last Edit/Upauthor: Ed Hamilton

[Y-DNA-projects] True Mutation Rates, by Tom Crago, in the Y-DNA-PROJECTS-L Archives,

a PDF file, Mutation Rates -- Who's Got the Right Values?, by Whit Athey, in Journal of Genetic Genealogy 3(2)i-iii, 2007,

Re: [Y-DNA-projects] True Mutation Rates, Clovis LaFleur, also in the Y-DNA-PROJECTS-L Archives,

and a post by Mr Kerchner in the Y-DNA-PROJECTS-L Archives, Male Line Specific Y-STR Average Mutation Rates -- the one size shoe/(haplotypemutation rate) fits all approach is not valid.

The Hamiltons

On my father's side our y-DNA matches on 37 of 37 markers with a descendant of Preston Fields, who was born about 1795 in the northeast corner of Tennessee. Preston's descendants were always told that either Preston or a male ancestor of his was actually a Hamilton, but had been brought up by a Fields family. For some background see the article Preston Fields of Pike County, Kentucky.

Our most distant paternal ancestor is James D Hamilton, who was born in 1818 in Cape May, New Jersey, a generation after Preston.

Let's assume a generation of father to son is 30 years. It was about that on my father's side, down to my cousin's son whose DNA was submitted. However on my mother's side, the generations were longer.

Remembering that Baltimore was a port of entry for the Scotch-Irish, let's say that Preston Fields' father ___ Hamilton, was born about 1765 in Baltimore and migrated to western North Carolina/Eastern Tennessee by 1795, where he met Preston's mother.

Now suppose our James D Hamilton's family was also from Baltimore, with his father being born there about 1788 and his grandfather about 1758. Now we are back to the same generation as Preston Fields' father.

But it is very unlikely that James' grandfather and Preston's father are the same person. He would have to have fathered a child and almost immediately left for the frontier leaving a family behind. Of course the Hamilton track record might actually suggest this as a probability, come to think of it.

Dismisisng that possibility for now though, the common Hamilton-Fields ancestor might not have been for yet another generation, born in about 1728-1735. We would have to go back at least eight generations to find our common ancestor with the Fields family, thusly:

his grandfather
..his father
....James D Hamilton
........James M
..............his son.

Add to that this interesting fact that I just had to throw in: There is a distance of about 600 miles between Knoxville TN and Baltimore MD, according to the "As The Crow Flies" Distance Calculator at http://tjpeiffer.com/crowflies.html.

The O'Rourkes

For some background, see The O'Rourke y-DNA -- an update, November 2015.

On our O'Rourke side, we match on 35 of 37 markers, generally accepted as a pretty close match, with a family who:

1) share the same surname,
2) used many of the same given names as our family from one generation to the next,
3) and is known to have lived within 200 yards of our family in Ballintur Townland, County Down, for 150 years.

On only 35 of 37 markers? If their DNA mutated at the same rate as the Hamilton's did, we should match on 38 of 37 markers. I mean the families lived side by side for probably 200 years or so. Can we possibly be less closely related to them than to the Fields?

We know that our O'Rourke family goes back 5 generations without being related to the other O'Rourkes:

James O'Rourke
........my cousin,

and I think there were the same number of generations involved in the other O'Rourke family tested.

Comparing the two

The Hamilton DNA did not change at all in 7 generations. If the rate were the same for the O'Rourke DNA, how far back would we have to go to get 2 mutations? Actually, an infinite number of generations, correct?

But let's say the Hamilton DNA mutated at a rate of 1 every 8 generations. In our case the mutation would have occurred just before the Fields line split from ours. Apply that to the O'Rourkes. We would go back 3 more generations before the first mutation occurred, then 8 more to arrive at the second.

Were the O'Rourke families living side by side in Ballintur for 330 more years? Since 1450? I doubt it. And don't forget 30 years per generation is on the short side for the O'Rourkes. We could be talking about the year 1400 here.

Next, suppose we found out that one of the O'Rourke mutations occurred between my uncle and my cousin? Then my uncle's y-DNA would have matched on all but one marker with the other O'Rourke family. Is he suddenly more closely related to them than we thought?  Obviously not.


The first conclusion is short and sweet: the Hamilton and O'Rourke DNA must have mutated at different rates. Or as one of the articles said, they mutated at the same rate, but the rate of repair for the Hamilton DNA is faster. Same difference as far as I, a humble layman, can see.

Secondary conclusion: take estimates of kinship based on y-DNA and autosomal DNA for what they are, estimates.

Finally, and most practially: we could be more closely related to the other O'Rourke families in Ballintur than the DNA results seem to indicate, and more like what the paper records themselves indicate.


The image of a DNA strand comes from Wikipedia:File:DNA orbit animated static thumb.png.