Recently, group administrators received information that Y matching has changed at Family Tree DNA.
This is a welcome update.
The new changes reflect less restrictive matching algorithms, reflecting knowledge gained about how mutations on the Y chromosome occur.
These new matching algorithms also affect the calculation of genetic distance. I wrote about genetic distance here, and this new information supplements the original article.
All changes result in less restrictive matching. Therefore, if you notice any changes at all, you should have additional Y DNA matches, not fewer, whether as a result of your own marker values of those of someone you now match, but didn’t before.
Normally, if person 1 has a value of 12 and person 2 has a value of 14, on any marker, the genetic distance is counted as 2, the difference between the two values.
The new changes vary from the normal matching, depending on the marker and the values.
Null Value Markers
When a marker has a null value, meaning a value of 0, that marker will be counted as one difference when compared to other markers with numeric values.
The new genetic distance calculation of 1, when one individual has a marker value of zero, has been implemented to reflect that the mutation resulting in the deletion of one individual’s DNA at that location likely happened in one step, not in several.
Null values are most often seen on marker 425, but can appear elsewhere as well. All null marker values are treated in this same manner.
Dual Value Markers
Most markers with hyphenated values are being treated less restrictively. Family Tree DNA has provided the list of markers affected by this change, below.
Matching now looks at the total difference of the two values combined, not the difference at each hyphenated value individually. In other words, the order of the values no longer matters.
There are two changes in the above calculation when any two values are the same.
- Change 1 – The common values cancel each other, regardless of where they appear in the marker.
- Change 2 – The genetic distance is now 1 if there is a difference in the remaining markers, instead of the previous 3, in this example. In other words, the value of 1 reflects that there is a genetic distance and does not assume that the mutation occurred in 3 discrete steps.
However, in the instance where any two values are NOT the same, a different matching routine is involved.
In this case, the genetic distance is 2 because there are no common values to cancel and the mutations are much more likely to have occurred discretely.
Marker 464 typically has 4 values, 464a, 464b, 464c and 464d. However, this marker can be found with from one to several additional values, such as 464e, 464f, etc.
In the event where the common marker values are the same, above, the fact that one person has additional markers, regardless of how many, is counted as one difference, because the mutation that created these additional markers likely happened at one time.
In the event where the common marker values are not the same, as shown above, common values are cancelled, with the nonmatching values being counted as one genetic step, the same as in the dual value marker example above. In this case, one genetic step is assigned for the 4 extra markers, and one additional step for the difference between markers 464b and 464c, for a total genetic distance of 2.
Thanks to Family Tree DNA for providing this additional information.