TWO HUNDRED CENTIMORGANS

Even low centimorgan numbers can be important. June 24, 2024

This is another follow-up to my previous posts called Eight Hundred Centimorgans and Four Hundred Centimorgans.  This one involves more distant (but still important) DNA matches.  You might be thinking, "Those will be distant relatives and I don't really care about those matches."  Not so fast!  I'll explain why even these lower numbers might be very important to your family history, and they might not be as distant as you think.

Remember, the number alone isn't the only factor when figuring out a person's relationship.  We also have to consider what generation they are in.  We know those big matches, like 400cM and higher, are probably going to be close family members that you know.  Let's look at some situations involving a DNA match in the 200+ centimorgan range.  You might be surprised at what those numbers can reveal.

Let's say you have a 250cM match named Joe.  If Joe has his family tree filled out to the great-grandparent level, you should probably see some surnames from your family.  Why?  Because a match in the 200 range will likely be a second cousin.  If you've read my previous posts about relationships, you know that second cousins have two great-grandparents in common.  If Joe has two great-grandparents in common with you, his match makes perfect sense, right?  What if you see nothing in common with your tree though?  Should you be concerned.  Maybe.  It could mean Joe has an incorrect great-grandfather.  Some people don't care about these things because they may never have known their great-grandfather and it's no big deal to them.

This can get a whole lot more interesting though.  Let's assume we're right and Joe's great-grandfather was really another man.  Joe should investigate this.  If one great-grandparent is wrong, he'll want to check his other matches and see that he's really matching to his great-grandmother's family.  It could lead to him discovering his grandfather, or even his father, is not who he expected.  Or, let's say this was his paternal great-grandfather and his last name was Smith.  Then, we discover his great-grandmother had an affair and passed her child off as Mr. Smith's kid.  That would be Joe's grandfather, and he'd still have the name, Smith, but not really have Smith DNA.  His whole family name would be a complete sham.  I wrote another article about that specific subject called, The False Family Line.  Check that out for more info.  Are you seeing why even these low matches can be important and sometimes quite surprising?  There's more...

You might be surprised by your DNa results.

What if this 250cM match, was much younger than you.  When you look at his tree, you see big age differences between his parents, grandparents, and your family.  You can assume Joe is a generation younger.  With each generation difference, the DNA level gets cut in half (not exactly, but it is often a number close to half).  For example, If Joe is a generation younger and matching at 250cM, we can assume his parent (let's say his father) will match you at around 500cM.  Now things get really interesting.  Let's call Joe's father, George.  What could George's relationship be if he's in the same generation as you and matches at 500cM.  Half first cousin is very possible.  It could mean that your grandfather and George's grandfather are the same person!  Now we're getting into some closer family members.  See how it can get really interesting, really fast?

What happens if Joe is much, much younger than you?  If he's two generations younger, we start to see more possibilities.  If Joe's father matches you at 500cM, and he is a generation younger, then his parent might match you at 1000cM.  Joe's grandparent could actually be your first cousin?  Crazy, eh?  This might reveal one of your parent's siblings had a secret child.  Joe's relationship could be first cousin twice removed.  Other relationships at this level include half first cousin once removed and other things start to become possible as the numbers get lower.  The relationships can start to get really tricky, so don't jump to any conclusions.  You'll want to really investigate if you think something isn't right.  I just want people to understand, if you see DNA matches at this level and they have family trees filled out showing several generations, and you also know several generations of your family, you would have to see some common surnames appearing.  If you don't, find out why.

You might think this type of thing is unlikely, but think again.  I've seen this type of thing happen many times.  I've been asked to find information about a person's great-grandfather and it led me to discover their father was not who they though he was.  I even go a step further in some situations.  I've looked at third or fourth cousin matches.  People in the 100cM range with great-great-grandparents in their trees should have surnames in common with you.  Never underestimate those low matches.  In fact, don't even think of matches in the 200s as "Low."  They are all worth investigating. 

Next I'll talk about why sharing your DNA match list is a smart thing to do.


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