How Relevant is Genetic Distance

[FredSpringer]

I have an uncle who tested 67 out to markers and has a genetic distance of 3 from another individual with a different surname that had also tested to 67 markers. There is actualy only one marker different, but the values for that marker differ by 3 units. FTNDA time predictor indicates that there is a 95% chance that there was a common ancestor within 8 generations.
Is testing to 67 markers sufficient enough to determine a relationship between the two individuals? Is there a possibilty that different clans would have similar dna results aside from adoption or infidelities?

[Jim Barrett]

Fred,

Did your uncle have any matches with his surname? What is his Haplogroup? If it is in the R1b1 group lots of unrelated people match because that is a very common Haplogroup. Has his data been loaded to Ysearch? If not it should be. Once it has been added let us know the user ID so we can see his data.

In addition to the reasons you have given for different surnames matching is many unrelated people adopted the same surname. Sometimes people changed their surname.

[lgmayka]

There is actualy only one marker different, but the values for that marker differ by 3 units.
...
Is there a possibilty that different clans would have similar dna results aside from adoption or infidelities?

Exactly 1 marker difference out of 67 is an impressive match indeed!

Please keep in mind that adoption was much more common, and much less formal, in previous centuries. If a widow was left with a young son, she had powerful social reasons to remarry as quickly as possible; and if her new husband was willing to give her son his own name and raise him as his own, perhaps that was so much the better.

[msc_44]

66/67 is a match like very close because even me and my dad are a 66/67 match we also are actualy only one marker different, but the values for that marker differs by 3 units so just because the surname is different that doesnt matter you should start looking for adoption records.

[FredSpringer]

[Jim Barrett Fred,

Did your uncle have any matches with his surname? What is his Haplogroup? If it is in the R1b1 group lots of unrelated people match because that is a very common Haplogroup. Has his data been loaded to Ysearch? If not it should be. Once it has been added let us know the user ID so we can see his data.]

There were a couple matches with his surname, however the closest match was with the other surname individual. His Haplo is I1A predicted. He is FXNRQ at Ysearch, but it appears that the other individual is not present at Ysearch.

My reason for asking is that I have been informed that a 66 of 67 match is not a strong basis for a connection unless the surnames match. That seems to me to ignore adoption, or other possibilities. This is why I was wondering how likely it is that diferent clans would have the same dna. I was of the belief that dna testing was a 99.99% verification of relationship to the exclusion of all other peoples. Perhaps I am just lost regarding understanding genetic distance.

I am assuming that at times in generations past, adopting someone was not as bureaurcratic a process as it is today.

[lgmayka]

My reason for asking is that I have been informed that a 66 of 67 match is not a strong basis for a connection unless the surnames match.

Whoever told you that is severely misinformed, or more likely trying to fool himself. 66/67 is an extremely impressive match. The 3-unit marker difference can occur at one time, as a multirepeat deletion; and in fact, my own project has two relatives who differ by a 3-unit deletion on one marker.

[Bob Swinea]

A 66/67 match is a very close relation. As has been posted earlier, I would start looking into "adoption" possibilities. In our Abernathy DNA group, several of us match 67/67 with a gentleman who has a different surname. No doubt we are all related but the question becomes where/how? Our best guess to date is in the Civil War era when [the only time we could find] both of our families were within a buggy ride of one another. I understand the Civil War left lots and lots of orphans and foundlings. Just a suggestion..

Bob

[K. Campbell]

Rethinking Non-Surname Matches

Fred,

I agree with the consensus. Here are my thoughts from a previous post.

http://www.familytreedna.com/forum/showthread.php?t=1811

- Kevin

[Jim Barrett]

[Jim Barrett Fred,

My reason for asking is that I have been informed that a 66 of 67 match is not a strong basis for a connection unless the surnames match.

As you already know there are a lot of people who disagree with that information. I believe you have a strong match.

If there was an adoption it may be impossible to find proof that it occurred. If they exist, I'd look for census records around the time your different ancestors were born to see if the other surname was in the same area at the right time.

Did a relative from another line have the second surname. Worse case the child may have come form someone who was just passing through and you'll never find a record of it.

[josh w.]

The different surnames issue comes up often. After reading the short description of the Bayesian analyses for MRCA I was left with the impression that a surname match was not required in the tables that FTDNA presents. That is, the calculations did not presume a surname match. Indeed if the surnames did match, fewer generations would be listed than are actually presented in the tables for time to the MRCA. The FTDNA staff confirmed my impression.

[jmknapp]

Mutation rate

The chance of a mutation in one of 67 locations, even in just one generation, is pretty high. Published mutation rates for a specific site are in the 0.0021-.00026/generation range [Zhivotovsky 2006]. Using .0026, there's about a 16% chance of a mutation in one generation across 67 markers! Looking back 8 generations, you could definitely expect one.

[MMaddi]

If you want to understand how impossible it is, or almost impossible, speaking in terms of probability, for two people who are unrelated to differ on none or one or two or a few markers, with 67 markers, the proper method is to take each marker's mutation rate and multiply them all. If you do that, you will get an infintesimal number that will convince you the two people being compared must be related within just a few generations, certainly no more than 5-10.