DNA Solutions: Using Y-DNA for Genealogy

By Diahan Southard Premium

Question:  I had my nephew do the Y-DNA111 test to trace our paternal line and try to break through a brick wall. How can I understand and use the test results?

Answer: First of all, great job pulling that YDNA test out of your genealogical tool box. Working on a paternal-line brick wall is a great way to use this testing technology.

To start making use of your test, first evaluate the match page at Family Tree DNA <>, which is the only major testing company currently offering Y-DNA tests. The most important column in this table is the first column, called Genetic Distance. If your nephew had taken the 37- or 67-marker test, you’d want this number to be three or fewer in order for him and his match to possibly share a recent common ancestor. At the 111-marker level, though, you can allow for up a genetic difference of up to five. Within these recommended genetic distances, you can usually assume that you and a match have a common ancestor at or before eight generations into the past.

But don’t take my word for it. You actually can see for yourself the time frame in which you should be looking for a common ancestor with your match. To see these statistical calculations, click on the orange Tip button under your match’s name on Family Tree DNA’s main match page. This will show you a statistical calculation with the likelihood that you and your match share a common ancestor within a given number of generations. For example, you’ll see that it’s 70.93 percent likely that you and your match on 65 out of 67 markers share an ancestor at or before six generations. I like to start checking the matches’ trees for the common ancestor at the generation that boasts a likelihood of at least 75 percent.

understanding y-dna test results

Your choice of the 111-marker test was ambitious, as you could’ve selected the lower 67- or even 37-marker testing option, and probably still found out what you wanted to know. But in the end, you can’t ever have too much information, and you might be glad you opted for 111 markers. Remember that these numbers of marker represent locations that are evaluated on the Y chromosome. The more markers you test, the more locations you evaluate—and the better the estimate you can receive about when you share an ancestor with a match. For example, if you match someone at exactly at the 37-marker level, you should look for your common ancestor somewhere between four and eight generations back. But if you match exactly at the 111-marker level, it is much more likely that your common ancestor will be found within four generations.

Because you’ve had this high level of testing completed, you’ll want to be sure to also look for matches at the lower levels of testing. To do that, use the dropdown menu in the Filters section of your match page. Whenever you use this dropdown menu, make sure you only look at the matches who’ve had this level of testing completed, not anything higher. In the table on the opposite page, where we’re viewing matches at the 67-marker level, you’d pay attention only to Bartholomew and Samuel, and not Inglebert and Roger. This is because the numbers in the Genetic Distance column refer only to how well they match you at the 67-marker level, and not at the 111-marker level. Inglebert might match you exactly at the 67-marker level, but have four differences from you at the 111-marker level. Always evaluate your match at the highest level of testing available.

understanding y-dna test results

The best thing to do next with your results is to join a surname project. These projects gather individuals with the same or similar surnames to collaborate on paternal line research. To find one, go up to the top menu bar, and choose Projects, then Join a Project. You’ll see a list of some suggested projects (sometimes it seems these are blindly chosen without rhyme or reason) but if you don’t see your surname on the list, scroll down and type it in the search box. One of the biggest benefits of joining a surname project is the assistance of an administrator. Often your project administrator will be able to help with understanding your Y-DNA results and how they fit in with the rest of the group.

If you’ve tried all of these ideas and come up empty-handed, try getting another descendant of your Y-DNA line tested. You want to test the most-distant relative possible. Testing second cousins or closer will usually not be as helpful as testing a third or fourth cousin. To find one, go back to your brick wall ancestor, and follow a different son’s male line down to the present until you come to a living male with the ancestor’s surname to be tested. This newly tested Y-DNA can help you verify your connection to this ancestor, reveal more matches, and give you a good Y-DNA baseline to move forward with.

From the January/February 2018 issue of Family Tree Magazine.