Y-STRs and Genetic Distance

Any man who has tested his Y chromosome has certainly looked at a chart something like the above. I say man because only men have a Y chromosome—and it carries the male sex gene, called the SRY gene—which is inherited only from their fathers. Because there is no female sex gene, a woman is a woman because she lacks SRY, and lacks it simply because she has no Y chromosome. A mother, therefore, cannot pass a Y chromosome from her father to her descendants because SRYs do not "skip a generation." It's as simple as that. Although other chromosomes come in pairs, the Y doesn't. Perhaps one might say it's an eternal bachelor because it has nothing with which to pair. This bachelor "reproduces" itself by cloning itself and passing the copies to his sons—generation after generation. This means a man's Y looks like his paternal 6th great-grandfather's Y. It is the same Y, even if it gets modified a bit every few generations. Due to European naming conventions, the Y generally appears to follow the surname.

The chart above is an example of Short Tandem Repeats (STRs). They're just that: short strings of DNA that are repeated in tandem to one another n number of times. For example, strings of TCTA appear in the Y chromosome at position DYS391 (column 4). In this case, they repeat 10 times.

SIDEBAR: DNA is comprised of four chemical bases, as follows, the names abbreviated to A, G, T and C. The above example, then, is a sequence of thymine, cytosine, thymine, and adenine strung together 10 times.

Here we compare two testers by determining their genetic distance:

Using simple subtraction, find the difference between the paired values and note the grand total, which is the genetic distance (GD). This example has a GD of 15 out of 12 markers. What does that mean? For testers to be of the same family, they should have a genetic distance no greater than 1 out of 12 markers. But even that can be misleading. At twelve markers, many of us match with people across several geographic regions and surnames. But that match often falls apart once more markers are tested. For this reason, The Cooley DNA Project requests that members order 37 markers. Some admins prefer to see 67 markers.

The upshot is that even if the two men represented above had the same surname, they're not related. The genetic distance is simply far too great.

Make no mistake about it, this is science. But all science needs interpretation, and interpretation is an art, especially with a science as new as genetic genealogy. This is particularly true of STRs for the very reason that they tend to be fickle. Those numbers can go up and then back down in a subsequent generation. That's why GD is so important. It looks at the overall trend in a group of related subjects. A trained eye, however, can sometimes spot patterns. But, alas, patterns can be gained and lost with additional data.