Here is a parable, and like all parables, the point is not if it really happened but the lesson we can learn from it. Once upon a time, someone asked Picasso why he could get away with making art the way he did, and other people could not. Picasso grabbed a piece of paper and a pencil and sketched the most beautiful and perfect picture of a horse. He told the person that once they could make things like that, they were free to break the rules and make art however they wanted.

This is also true in the genealogy world. There are a lot of rules about how to go about doing good genealogy research, especially in the genetic genealogy world. If you want to be successful when you bend or break these rules, you have to know why those rules exist in the first place. I’m going to be talking more specifically in this post about why the Gedmatch settings are the way they are, and why it’s probably not a good idea to change them.

The first setting that people often fool with is the minimum segment size. The default is 7 cM.

I’m hoping everyone knows that a centimorgan (cM) is a unit of measurement used to measure DNA. If not, go read this post here and come back. So why is 7 the default, and why is it a bad idea to lower it?

I talked about identical by descent and identical by chance in my post DNA in a nutshell. I also provided a link to a really good chart that shows the odds that a segment is there by chance or by descent. Once you get below 7 cM, it’s only 50/50 that you share a segment with someone because you’re related to them, and not because you randomly happen to share similar DNA. It’s like comparing Anna Karenina and Hop on Pop and deciding that since they both have the word “the” in them, they must somehow be related. As someone in one of the genetic genealogy Facebook groups said, “lower the settings too much and you’ll match a banana.”

Can this rule be broken? Yes, but the results should still be used with caution. Sometimes people are trying to force a connection that just isn’t there. Perhaps they are worried for reasons covered in my post Is this normal? I’ll repeat (as many times as needed), once you get past second cousins, it is possible not to share DNA with a cousin. It doesn’t mean anything is out of the ordinary. I explain why in my post We all have two family trees. If you really, really want to find a matching segment with a cousin because they are on your brick wall line, you are more than welcome to try, but the burden of proof still rests on you to prove that that segment actually came from that ancestor that you share, and not because you randomly happened to have the same DNA in that spot (the odds are more in the favour of randomness). You might be pursuing this segment in this spot as if it is a real match and assume that whoever matches you there must be from the line you share. You could invest a lot of time and energy pursuing this match, only to find out that you’ve been on a wild goose chase. But hey, I pursue matches that are likely to be outside of the genealogical time frame, so I guess we’re all allowed to be masochists in our own way.

The other settings (there are actually a few of them) are for SNP counts. I don’t know why anyone would want to change these since very few people actually know what SNPs are (most people pronounce it like ‘snips’). Here’s what I know about SNPs. DNA testing does not test all our DNA. That kind of testing (although available) is still pretty expensive and it’s unknown whether doing so would provide us with better results. In my post Are DNA tests accurate? I wrote a how 99.9% of the DNA people share is identical (we also share 60% of our DNA with a banana, so I wasn’t kidding about lowering the settings too much). What a DNA test does is look at specific points where the DNA is likely to be different. These points are called SNPs. Some parts of our DNA have more of these points than do other parts. Therefore, if you know which parts are SNP-rich, you could consider lowering the cM value while raising the SNP value to compensate.

Or maybe, just maybe, you could consider leaving the settings where they are.