It may not look like much, but the two bits of shared DNA shown in blue show that I am at least this person’s fifth cousin. (Click the image for a larger version).

My latest dive into my DNA began with an email. Someone contacted me via 23andMe and said that we may be fifth cousins and asked if I would like to compare genomes.

Ok I thought, why not? And so I contacted them. We compared our genomes and this is how much DNA we had in common. Not exactly awe-inspiring is it?

Even though it looks pretty wimpy, it is actually a couple of good sized chunks of DNA we share. Odds are that I am indeed related to this person. To understand why, we need to step back a bit and remember how DNA is passed from generation to generation.

As you undoubtedly know, we get half our DNA from our moms and half from our dads. This DNA is passed down in the form of chromosomes.

What you may not know is that except for the Y chromosome, we don’t inherit an exact copy of our parents’ chromosome. Instead we inherit a mix of each of their pair of chromosomes. Clear as mud as usual, Dr. Starr…

We all have two copies of each of our chromosomes, one from mom and one from dad. So we have two copies of chromosome 1, two copies of chromosome 2 and so on up to chromosome 22. The chromosome 1 we get from mom is actually a mix of her two chromosome 1’s. Same thing with our chromosome 2 and so on.

This mixing happens with each generation. I have tried a couple of times to explain this in words but I got tired of writing chromosome over and over. So instead here is an image that I think explains things a bit better than words:


The image focuses on a single pair of chromosomes. It starts out with the chromosomes of your four grandparents (G1-G4). G1 and G2 pass a chromosome down to mom and G3 and G4 pass one down to dad.

As you can see, the chromosome that was passed down didn’t exist before. It is a unique mix of each grandparent’s pair.

The same process then happens with mom and dad. More mixing and matching happens leading to your unique pair of chromosomes.

Note that the chunks of DNA from your grandparents get smaller as they head down the family tree. Your children would inherit even smaller chunks.

Over the generations, some chunks would become so small that they could no longer be easily linked to the originals. And eventually, some ancestors’ DNA would disappear entirely from the record.

This is why the fact that this woman and I share a couple of chunks of DNA is so significant. The DNA of fifth cousins needs to travel down two different branches of the family tree from great, great, great, great grandparents. The only reason we have any chance of seeing the relationship at all after all this traveling is because we have 23 pairs of chromosomes.

This kind of analysis is brand spanking new and wouldn’t have been possible even a few years ago. Back then, we could only look at little snippets of our DNA. These old school tests have trouble telling if two people are cousins let alone fifth cousins!

So now we can find a bunch of folks related to us. We each have 64 great, great, great, great grandparents which means we have tons of fifth cousins.

And now I run into a common problem I have with genetic tests like this…so what? I can find lots of relatives but is there anything I can do with that information?

I certainly don’t know who any of my 64 great, great, great, great grandparents were. If my grandfather was born in 1906, my great, great, great, great grandparents were born around 1800 or so. I don’t know anything about my family tree from way back when and I am not sure knowing I am related to Jane Doe from Poughkeepsie helps me figure it out.

I suppose that as more and more of us plumb the depths of our DNA we’ll be able to better trace these sorts of relationships. But in the meantime, this is probably only useful for the real genealogy hounds out there. Since I’m not one of these folks, I wonder how useful it is even to them…

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Finding Fifth Cousins 29 March,2010Dr. Barry Starr


Dr. Barry Starr

Dr. Barry Starr (@geneticsboy) is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition.

You can also see additional posts by Barry at KQED Science, and read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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