Why Use a Chromosome Browser

A chromosome browser can help you understand how you are related to your DNA matches—and it’s a very cool visual!

 
chromosome browser DNA matches.png
 

Before we start, let me be very clear: 98% of the time, you do NOT need a chromosome browser to do successful genetic genetic genealogy work. Most of the time, it doesn’t matter how you are sharing DNA, only that you are sharing.

OK, now that I got that out of the way, let’s get down to using these things for a little insight, and a lot of cool science stuff.

Most of the major DNA testing companies have chromosome browsers. Though they may display the data a little differently, all of them show physical locations on each of 22 chromosomes where you and your genetic matches share DNA.

What do chromosome browsers look like?

Here’s what the results looks like on Family Tree DNA for a man whose DNA is being compared to three of his great-grandchildren. Where he overlaps genetically with each of them is shown in blue, red and teal, one color for each of the three great-grandchildren.

 
chromosome browser FTDNA.png
 

(The light grey are non-matching areas, and the gray and black striped sections represent areas that aren’t included in the testing analysis.)

Why use a chromosome browser

There are three reasons you might want to take a peek at a chromosome browser. First, pictures are more impactful than numbers. Second, you can get a better idea if you and your match share a recent common ancestor. And third, you can dig deeper into discovering a shared ancestor.

Put it in a Picture

To me, it’s mind-boggling that I can actually see a detailed physical breakdown of the DNA data I share with a relative. This is one of my favorite ways to use a chromosome browser: to see tangible evidence of my specific biological connection to a cousin—and to know that this exact piece of DNA was once walking around in one of our shared ancestors. (This is a fun way to get your relatives more engaged with their own DNA test results.)

Pieces vs bits (and the problem with glitter)

Another way to use a chromosome browser is to get a sense of how closely you may be related to your match. The total number of shared centimorgans only tells you how much TOTAL DNA you share. Those who are more closely related will share longer individual pieces of DNA, as opposed to the many, short bits shared by those who are much more distantly-related or who simply may share a lot of DNA because they come from endogamous communities.

You can think of these short bits like ancestral glitter. My husband HATES glitter. He has tried to ban it from the house, but when you have a little girl, glitter just comes on everything. Glitter is persistent, it lasts for months on sofas and carpets and, well, everywhere. You need to be sure the pieces of DNA you see showing up are evidence of a real—and recent—relationship, not just leftover glitter.

In general, you want shared pieces of DNA over 10 cM. Anything smaller is likely just glitter.

Dig Deeper

Some genealogists use chromosome browsers to identify groups of people who all share the same pieces of DNA (triangulated groups, or TGs). Let’s say you and a couple of known closer cousins (like first or second cousins) share the same piece of DNA. That often means you got that shared piece of DNA from your common ancestor or ancestral couple.

Now let’s say you add a mystery match to see if he or she also shares that same piece of DNA. IF they do share, you may conclude that this new match is related to that ancestral couple you know you share with your close cousins.

It’s a good thought and may sometimes work. But BE CAREFUL.

Using this technique with more distant matches, like third and fourth cousins, can be problematic. The more distant the shared ancestral relationship, the more unlikely that multiple cousins share the same piece of DNA. Shared DNA between multiple fourth cousins is less likely to point to a single recent ancestor and more likely the result of belonging to a shared population group (see that earlier link on endogamy). For example, perhaps you may all share Ashkenazi Jewish roots, and you’re looking at a very “Jewish” piece of shared DNA.

Chromosome browsers are available at 23andMe, Family Tree DNA, MyHeritage DNA, and are promised to be coming soon for LivingDNA customers. Here’s a more detailed article on the MyHeritage DNA chromosome browser, and some additional (but more complicated) ideas for using chromosome browsers to place matches on your genetic family tree.

What to read next: this excellent two-part series on pedigree collapse, which includes the math you need to see how this phenomenon may be affecting your genetic relationships.

 

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