Marker off by 2?

[James B. Barnes]

My third cousin and I both took the 25 marker test. We were a 24/25 match. I was surprised that at marker 464C I was a 17 and he was a 15.

I was wondering how common this was. I was expecting we would be off by no more than one at any marker location.

James B.

[Bill Hurst]

Hi James,

In our Hurst project we have two cases where markers had three-step changes between fairly close cousins. We know that the change is all in one of the persons. We believe, and FTDNA agrees, that the three steps happened in one event, rather than there being three one-step mutations. I've never seen a paper explaining this.

Bill H.

[James B. Barnes]

re: Marker off by 2?

I hadn't imagined that. I had wondered if they had made a mistake. I was told that certain marker locations were more active than the average (that's my layman's explanation).

I wonder how frequently this happens and if it is typically the same marker locations that have this behavior.

James B.

[Bill Hurst]

James, our two three-step mutations were at 464a and 385b. The phrase FTDNA used to describe the second of these was that it "should be considered a single evolutionary event, rather than the traditional 3 points away...."

Bill H.

[James B. Barnes]

I'm now realizing that the science of studying DNA is presently not as "cut and dried" as many of us would like. I think we have to accept the fact that scientists are learning more every day, making new models to describe the results they are seeing in DNA tests.

This was the response I recieved from Bennett Greenspan regarding our 24/25 match which included one two-step mutation. The web-site he cited offers a good scientific explanation of the situation, but it is very technical.

The complex marker 464 is our fastest moving marker and subject to both
single and double step mutations. It there follows the Infinite allele
model rather then the Step wise model for more on this please see:

http://nitro.biosci.arizona.edu/ftdna/TMRCA.html

I'm now less skeptical about our results and it gives me a deeper appreciation of how careful we need to be as we try to interprete the results from our DNA tests.

James B.

[cjms]

Since I am no scientist, I tend to get rather confused, to say the least. In our project, 3 lines turne dup a 12/12 match. Two of those lines were known 3rd cousins, the last line (mine) had no known connection with the first two, other than the common surname. We extended the testing for my line and one of the matching lines and came up aith a 24/25 match on the 25 marker test. We have a 1 step difference (18/19) in marker 448. Neither of us are sure if this narrows the gap in the distance to our common ancestor! I just begin to think I'm getting it, then something confuses me again. I feel a bit lacking as the group administrator to say the least!

[James B. Barnes]

Congratulations! It sounds like DNA testing is working for you. Hope you're able to figure out how you are connected to the other two. I don't think being off by one will give you a meaningful idea of how many generations back your MRCA was. But... I think you can start researching each other's lines with full confidence you have a common ancestor waiting to be found.

Good luck!

[bob_allison]

Genetic genealogy, relying as it does on chance mutations, will never be "cut and dried". It can only indicate probabilities - some may be close to 100% or close to 0% but cannot indicate certainty.

Some events, such as brothers differing on several markers tested, may be very, very unlikely but since there are several billion people in the world it is likely to happen in some cases.

Similarly two unrelated people in the past with different y-dna can have descendants whose dna mutates over a long period such that they exactly match today.