18 October 2013

When the Male Line Fails-Where Did the Chain Break?

If a male line DNA test indicates two researchers are not related, does it mean that their supposed direct line male ancestors are not related? It seems to me that is not necessarily the case. I'd be interested in thoughts on this little scenario.

The Oldmans

Kevin Oldman has researched his ancestry back seven generations to Henry Oldman, who appeared in Kentucky in 1820 from nowhere.

Bubba Oldmann has researched his ancestry back seven generations to Thomas Oldman, who also appeared in Kentucky from nowhere in 1820.

Thomas and Henry Oldman lived in the same Kentucky county in 1820 and 1830, but their listing on those two population census schedules are the only documents on which they appear together. Their farms are in the same township, but not adjacent. After 1830, Henry leaves Kentucky for Indiana. Thomas dies in Kentucky. No record ties them together other than the census. There's no hint they were related other than their enumerations in the same census district in 1820 and 1830.

Kevin and Bubba learn of each other through the internet and discover that they are potentially related through a common Oldman ancestor. They speculate that Henry and Thomas are most likely brothers or first cousins.

Kevin and Bubba decide to have a y-DNA test performed to determine if they share a common male ancestor. The test indicates that Kevin and Bubba do not have a common strict paternal line ancestor. Kevin and Bubba conclude that Henry and Thomas are not related based upon the test results. They continue to compare research notes, but are left convinced they are researching different Oldman lines. After all, if Henry and Thomas were related through a paternal ancestor, it would have showed up on the y-DNA test Kevin and Bubba took wouldn't it?


There's something Kevin does not know.

Kevin's ancestor, Clueless Oldman (son of Henry), thinks he was the father of all the children his wife had. Clueless is not aware of the fact that during his marriage his wife had an ongoing relationship with an unmarried neighbor. During her lifetime, the wife was relatively certain that the neighbor was actually the father of two of her children and not Clueless. One of those children Clueless' wife had with the neighbor is Kevin's Oldman ancestor.

So "technically" Kevin's not really and Oldman at all--that's what the paper trail says, but it's not the biological reality. Kevin is not actually a descendant of Henry Oldman.

And it turns out that Henry and Thomas Oldman were indeed brothers.

Bubba Oldman and Kevin Oldman are not related--because Clueless Oldman didn't father all his supposed children. And that has nothing to do with whether or not Henry Oldman and Thomas Oldman were brothers.

The Inverse

Readers with a mathematical or logical bent, may notice that there's a logical fallacy at work here. The original statement is:

  • If Kevin and Bubba share a common ancestor, then Thomas and Henry are related. 
This is a valid statement based upon DNA theory. If Kevin and Bubba share a common male ancestor, then Thomas and Henry share a common male ancestor. But the statement:
  • If Kevin and Bubba share a common ancestor, then Thomas and Henry are not related.
is not true and cannot be derived from the original statement. It is the inverse of the original statement and the believing the that the inverse is true is referred to as the "fallacy of the inverse."

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