LESSON 03: Exploring the Y Part 1

Thanks to Angie Bush for her input in this Lesson!


The Y-chromosome is linear, and contains approximately 59 million base pairs with STRs and the SNPs interspersed along its length. In this lesson we will cover STRs and in the next SNPs. There are very few genes on the Y-chromosome and these are also distributed across the length of the Y-chromosome. This link is to a browser showing where STRs, SNPs and genes are distributed along the length of the chromosome.

Remember only males can take an Y-STR DNA test as only males have a Y chromosome. This is the test used for surname projects and for those hoping to determine their relationship to other men with the same surname. It is also the test that male adoptees should use for determining the possible surname of their biological fathers. Y-STRs (pronounced Y "stirs") test what had previously been referred to as "junk" DNA because we didn't know until fairly recently that it had a purpose. We now understand that it is used in directing the turning on and off a various other DNA controlled mechanisms. Y-STR stands for the Y chromosome and STR is short tandem repeat. The tandem repeat refers to a series of DNA that is repeated a certain number of times. The number of times this is repeated varies among men and gives us a value for each Y-STR marker we test. A marker is simply the generic term we use when referring to a STRThe results looks very simple:

DYS 393 = 13

Translation: the marker called DYS 393 has a value of 13, which means the sequence (bases in a particular order such as: TATT) is repeated 13 times: 


A set of these marker values gives us a unique signature, also known as a haplotype. A Y-DNA 37-marker test will give a list of 37 markers and their values. When looking at a list of Y-DNA results at first they appear to be all fairly similar. But a closer look will yield patterns.

Here is a screenshot of part of the Wheaton surname project results page at FTDNA which I administer but most projects will look very similar.

The first four columns are self explanatory (Kit Number, Name etc---we will get to Haplogroups in the next lesson). The fifth and subsequent columns labeled DYS363, DYS390 etc. refer to the STR marker's name. If you look closely you will see that the results are clustered into Groups denoted by a horizontal colored row and a group description. If we look at the two kits in Group "C" we can see that the values are identical except for the first value at the CDY marker (slide your horizontal scroll bar all the way to the right if you cannot see). So for the CDY value the Kit #232717 has a different value (mutation) than the one below it.

A mutation is simply a change in value due to a replication error. So in our example instead of 39 repeats Kit# 232717 dropped a repeat so all of his direct male descendants will carry a value of 38 at CDYa. Lest you think replication errors are bad they are the way in which we can trace how closely two men are related. Some markers are more prone to mutations than others. You will note that some of the columns are blue and some are burgundy. The faster mutating markers are shown in burgundy. So differences in values at these markers are not as significant as those at the slowest mutating markers (blue). Mutations do not happen very frequently but they do happen randomly from time to time. They also happen with slightly higher frequency the older the father is at the birth of his son. You will note that values are shaded light pink, dark pink, light blue and dark blue. If the value is one below the modal (average) it will be light blue, more than one below is darker blue. If the value is one above the modal it will be light pink, more than one above dark pink.

Let's take a closer look at Group "B" (above). All of the men in group B are related in a "genealogical time frame." This genealogical time frame refers to the period since surname adoption, roughly since about the 1400-1500's. You will note that several of the men do not bear the surname Wheaton. Remember when I said DNA doesn't care what we want it just reports what is there. We do not know which surname came first but what we do know is that all of these men are descended from a common ancestor on the patrilineal (father's father's father's line). More on how this can happen on the page Dealing With The Unexpected Result.

This screenshot shows you what a group of men who are not related on the "Y" looks like. Note that the haplogroups (I1, R1b1a2, G) are different and the values differ greatly even though some values at some markers are the same. These are men who participate in the Wheaton project via their atDNA but they do not bear the surname Wheaton and therefore do not match any of our Wheaton profiles.

If you are a male you may wish to see your relationship to other males with your surname. Perhaps there is a legend in your family about three brothers and they are all purported to be descendants of a particular man in Washington Co, VA. A Y-STR DNA test could prove or disprove that story. Perhaps you do not know where your Jones family came from. A Y-STR DNA test might connect you with a cluster of Jones from Glamorgan, Wales or a group of Jones from County Cavan, Ireland or even a group from Telemark, Norway who were originally Johansson and became Jones upon their arrival in the US.

Y-STR DNA testing is fairly straightforward and easy to see. I suggest that you join the group at FTDNA for your surname check here or if you are adopted you should join the FTDNA Adoption Group. You should also consider joining haplogroup and/or location specific groups. Most groups have helpful administrators and all offer some discounts on kit prices. If there is not a group for your surname you can do what I did and start one.

There are also Y-STR projects at ANCESTRY but there aren't as many and FTDNA is currently the gold standard for Y-DNA. It is valuable to check Projects or surnames at ANCESTRY but their interface is much harder to use. Unfortunately the markers are listed in a different order than FTDNA so you will need to account for that difference. Also check the Y-Search database and Soresnson Y-database (links embedded and below).

Additional Resources

Y-STRS Standards from Sorenson
Y Chromosome Testing 101 from Blair DNA Project. Very nice expansive info.

ISOGG Y Browse  Y-STR and Y-SNP markers and their loci by Thomas Krahn

LESSON 04: Exploring the Y Part 2
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