Several of our markers are multi-copy markers, which tend to undergo change more rapidly than other markers.
What is a multi-copy marker?
A multi-copy marker is one where several copies of the marker exist on the Y chromosome. The names of a multi-copy marker include small letters, such as a or b, following the marker DYS name.
When selecting the markers for its various tests, Family Tree DNA includes one or two multi-copy markers in each panel, corresponding to the four YDNA tests available. The 12-marker YDNA test has one multi-copy marker (385). The upgrade to 25 markers adds two multi-copy markers (459 and 464), and the upgrades to 37 markers and then to 67 markers each include two additional multi-copy markers. Inclusion of these multi-copy markers is important, based on both scientific attributes of the marker as well as the genealogical implications.
Test Multi-Copy Markers
12 Marker 385a, 385b
25 Marker Upgrade 459a, 459b and 464a, 464b, 464c, 464d
37 Marker Upgrade YCAIIa, YCAIIb and CDYa, CDYb
67 Marker Upgrade 395S1a, 395S1b and 413a, 413b
For markers to have value in genealogical research, they must be stable -- but not so stable that they can't differentiate lineage. They also should change, but not change so quickly that closely related persons don't match. A well-formed panel includes a range of markers that change relatively rapidly and markers that change relatively less rapidly.
Why have I re-interpreted or re-coded results from our CDY marker?
The CDY marker is present in two copies on the Y chromosome -- at two different locations. For illustrative purposes, let us call these two locations CDY1 and CDY2. For the presumed Gossett patriarch, these two copies of the CDY marker have different values -- one has an allele length of 38, and the other, 39. However, the analytical procedure (i.e., the primer pair) used to measure marker-values is not capable of determining which location has which value. All we know is that one has value of 38 and the other, 39. The standard convention for reporting multi-copy-marker results is to report them in order of ascending value, and to put lower-case letters after the DYS name. Thus, FT-DNA would report the two CDY-marker results for our patriarch, in ascending order, as "CDYa = 38; CDYb = 39." Let us then arbitrarily assign these values to the CDY1 and CDY2 locations, respectively. (That is, we shall name the location where allele-length is 38, CDY1, and the location where allele-length is 39, CDY2).
Suppose, then, that a participant's line incurs a -2 change at the CDY2 marker location (i.e., its value drops from 39 to 37). FT-DNA's analysis would detect values of 38 and 37 for the two copies of CDY, but -- not knowing which location (CDY1, CDY2) possesses which value -- reports the result in ascending-value order as "CDYa = 37; CDYb = 38." If you do not realize what's really going on, this superficially appears as though both markers have changed -- the first marker dropping from 38 to 37, with the second marker dropping from 39 to 38. That's because, in this ascending-order reporting convention, this new CDYa result now actually corresponds to the CDY2 location, whereas in the patriarch the CDYa value corresponds to the CDY1 location. This screwy situation arises because there is no way to attribute result to actual location. However, a logical human being can intervene to make the correct attribution: It is unlikely that the (CDYa, CDYb) change from (38, 39) to (37, 38) occurred via changes in both copies, but rather was the result of a -2 change in one of the copies -- i.e., CDY2.
All of this suggests caution in the interpretation of marker-changes where multi-copy markers are involved. You need to be aware of the ascending-order reporting convention that is employed and adjust accordingly.
The scenario described above is exactly the scenario exhibited in Phil's lineage. I therefore have reinterpreted the reported CDYa, CDYb results, converting them to a location-specific result by invoking a marker notation (CDY1, CDY2) that attributes results to specific locations on the Y chromosome. It's a more accurate way to view the results for genealogical interpretation.
Here's another example. Consider the following results from a four-copy marker (such as marker 464a,b,c,d):
Individual A: 15 15 17 17
Individual B: 13 13 15 17
At a glance, you may think you see three differences, but there are really only two -- i.e, most likely there were changes in only two of the marker-copies (a 15 changed to a 13; and a 17 changed to a 13). What appears to be other changes is merely the result of the ascending-order reporting convention.
In summary, I've re-coded CDY-marker data to properly reflect their genealogical significance. As more data come in, I might have to do similarly with other multi-copy markers.