23/25 Match Same Surname

[EQueen]

Please forgive elementary questions; but I'm still puzzled.

Probable siblings were xdoor neighbors in 1830. Same surnames.

23/25 markers match.
DYS458 mis-match by one point
DYS447 mis-match by one point

"Interpreting Genetic Distance" reports that "results show mutations, and therefore more time between you and the other same surnamed person". Probably related.

Yet, report from FTDNA was that there is a 99.9% probability of a relationship.

Question: What is the % probability of a common ancestor at 8 generations back?

Thanks. Gene

[Jim Denning]

very good question!!! ps no question is too stupid
i have my greek 24/25 who is now 37 distance of 8 i figure 2500 or so years but does that have to be 2500 yrs ago or 150 yrs ago

[jablair]

Question: What is the % probability of a common ancestor at 8 generations back?

Gene,

If you assume the typical mutatation rate of .002 (1 mutation in 500 generations) the probability of 2 mutations out of 25 markers in 8 generations (16 transmission events) is 14.379%.

If you assume a faster mutatation rate of .003 (1 mutation in 333 generations) the probability of 2 mutations out of 25 markers in 8 generations is 21.686%.

I would quess the actual probability lies somewhere between these two figures.

John

[EQueen]

23/25 Match Same Surname

Thanks Jim and John.

I'm still moving cautiously with assumptions; thus please confirm or correct my understanding.

If the probability of 2 mutations out of 25 markers in 8 generations is between 14 - 22 percent; then the probability that there is not a common ancestor in 8 generations is between 78 -86 percent?

It appears that a logical next step is to upgrade to a 37 marker test to see if the odds improve. The upgrade is already in progress. However, using FTDNA's "Interpreting Genetic Distance..." explanations for the 37 marker test, even if there are only 2 mutations at that level, the two donors are neither "very tightly" nor "tightly" related; but are merely "related".

I assume that the mis-match of the 2 markers at the 25 level will remain as is, where is, and that there will either be additional mutations reported or the result will remain at 2. Yes/no?

Logical (or perhaps very illogical?) semi-final conclusion: There is a much greater than 50% probability that the next door neighbors in 1830 were not brothers?

Thanks again to all,

Gene

[jablair]

Gene,

You mention the year 1830 and 8 generations. There are 2 terms commonly used when calculating the estimated time to a common ancestor.

The first term is "generation". This is basically the number of ancestors between you and ancestor in question. You are one generation from you father, two generations from your grandfather, etc.

The second term used is "transmission events". To calculate transmission events you count the number of generations each participant is from the common ancestor and add them together. If you are 5 generations from the "common ancestor" and the participant you are comparing with is 4 generations from the "common ancestor" then the total transmission events is 9.

When you say 8 generations back to 1830 do you mean "generations" or "transmission events"?

John