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Proving John Locke

Reader James Hawk made an exciting find — he might be related to 17th century English philosopher and physician John Locke.

That is Definitely Cool. In capital letters.

LockeLocke — born in 1632 in Wrighton, Somerset, England — was a political theorist whose early medicine training led him to strongly support the empirical approach in the Scientific Revolution.

And his political views were highly influential in early America: “His political theory of government by the consent of the governed as a means to protect ‘life, liberty and estate’ deeply influenced the United States’ founding documents. His essays on religious tolerance provided an early model for the separation of church and state.”1

His notions of social contract and the natural rights of man didn’t make him popular with the British monarchy and he ended up exiled from England for a time. He eventually did return to the country of his birth, and died there, in Essex, in 1704.2

Definitely Cool.

So, James asked The Legal Genealogist, “Can you suggest the best test that could prove this?”


Not so cool.

Let’s start with the fact that Locke himself never married and had no known children.3 So it can’t be direct descent from Locke.

And Locke’s only sibling, his brother Thomas, isn’t known to have had any children either,4 so it wouldn’t be direct descent from Locke’s father. But Locke had cousins on his father’s side, so…

Now, a man named Hawke isn’t terribly likely to be a direct male-line descendant of the Locke family. So realistically we can’t test for YDNA — the kind of DNA that is passed down only in a direct unbroken male line from father to son to son.5

To test across gender lines — where we have, say, a son of a daughter of a son that we need to test against, say, a daughter of a son of a daughter — we have to use autosomal testing: looking at the kind of DNA we all inherit from both of our parents.6

And that runs us right into the fact that autosomal DNA is going to eventually punk out on us.

It’s simply a fact that we inherit that autosomal DNA in a mix that changes, in a random pattern, from generation to generation in a process called recombination. In time, so much mixing occurs, in such random patterns, that we simply don’t receive enough DNA from any one ancestor for it to show up as a match to that same DNA in a cousin who descends from the same ancestor.7

So for autosomal DNA in particular, that thing called time to MRCA — the time to the most recent common ancestor — is a critical factor. If it’s just a short time, just a few generations, we’re fine. When it gets back in time, back many generations, our chances of matching someone else who might share that same ancestor get to be … well … remote.

How does that play out in this case?

Remember that we’re talking about getting back to John Locke’s grandparents — the most recent possible common ancestors here. Each generation can be calculated as roughly 25 years. Some of course will have been longer and some shorter, but that’s a general average often used to calculate the time to MRCA.8 With Locke himself born in 1632, figure his parents would have been born around 1607, and his grandparents around 1582.

And let’s figure that we’re likely talking about a genealogist who’s .. um … shall we say … well-seasoned … say, 50 years old.

So… (edited for clarity🙂 here’s what it means to try to prove that our genealogist shares a common ancestor with a specific other person living today, understanding of course that there may be a lot of candidates for that final column on the right:

Generation Birth year (est.) Shared DNA Chance of matching
person sharing ancestor
Genealogist 1965
Parents 1940 50% ~100%
Grandparents 1915 25% >99%
Great Grandparents 1890 12.5% >99%
2d Great Grandparents 1865 6.25% >90%
3d Great Grandparents 1840 3.125% >50%
4d Great Grandparents 1815 1.56% >10%
5th Great Grandparents 1790 0.78% Remote*
6th Great Grandparents 1765 0.391% Remote*
7th Great Grandparents 1740 0.195% Remote*
8th Great Grandparents 1715 0.098% Remote*
9th Great Grandparents 1690 0.049% Remote*
10th Great Grandparents 1665 0.024% Remote*
11th Great Grandparents 1640 0.0122% Remote*
12th Great Grandparents 1615 0.0061% Remote*
13th Great Grandparents 1590 0.0035% Remote*
* Remote = Less than two percent

See what I mean? The time to MRCA here is more than 425 years — likely as many as 15 generations. Even if we figured an average generation at 33 years, we’re still talking a dozen or more generations.

And here we start getting into the remote or no-realistic-chance-at-all range some 200 years before we’re going to hit that MRCA.

I’m not going to go so far as to say it’s absolutely impossible to gather evidence of common descent using autosomal DNA back that many generations.

I will say, though, that it’s the longest of long shots, and in this reader’s shoes — even though I am an unabashed fan of DNA testing far and wide — I’d focus my resources on pure paper trail research rather than putting my money on DNA testing here.

The time to MRCA just isn’t in our favor in cases like this.


Image: John Locke, undated lithograph, Library of Congress Prints & Photographs Division

  1. John Locke,” ( : accessed 21 Nov 2015).
  2. John Locke Biography,” ( : accessed 21 Nov 2015).
  3. New World Encyclopedia, ( : accessed 21 Nov 2015), “John Locke.”
  4. H.R. Fox Bourne, The Life of John Locke, 2 vols. (New York : Harper & Bros., 1876) I: 82; digital images, Google Books ( : accessed 21 Nov 2015).
  5. ISOGG Wiki (, “Y chromosome DNA tests,” rev. 13 Aug 2015.
  6. See Judy G. Russell, “Autosomal DNA testing,” National Genealogical Society Magazine, October-December 2011, 38-43.
  7. Ibid.
  8. Note that this is an open issue, with estimates ranging from 25 for females to 33 for males. See generally ISOGG Wiki (, “Generation length,” rev. 15 Jan 2015.
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