GENEALOGY-DNA-L ArchivesArchiver > GENEALOGY-DNA > 2002-11 > 1037816689
From: John S Walden <>
Subject: Re: [DNA] MRCA calculations
Date: Wed, 20 Nov 2002 13:24:49 -0500
Below is my take on your testing issues
At 10:15 PM 11/19/2002 -0500, you wrote:
>I see a consensus emerging here that on a 25-marker test (such as that
>offered by Family Tree DNA) any less that 22 matches is considered
There are no YES vs NO exact point in the Y-DNA test because of the
random nature of the mutations.
> In a study I am involved in, we have one case of a 21/25 match
>between two individuals with a solid paper trail back to a common ancestor
>(a total of 15 transmissions).
For a 25 marker test and 15 transmissions and .002 rate of change
the expected mutations are:
1 35 %
The reason FTDNA say 3 or more mismatches is that statisticians like the
for their accept/reject point. In your case a 22 marker match has only a
3% chance of being
> Two of the differences, however, are on DYS
>464. These are the readings:
> Subject A Subject B
>464a 15 17
>464b 17 17
>464c 17 18
>464d 17 18
DYS464 are 4 different markers but they all come out on the same
test so what is a,b,c, or d cannot be determined. They are just
reported in ascending order.
But in your we can "theorize" a haplotype and count the mutations to
your A and B
ancestor Subject A Subject B
464a 17 15 17
464b 17 17 17
464c 17 17 18
464d 17 17 18
Steps 2 2
No matter how you look at it and which starting point you pick there
are at least 3 mutations and 4 steps involved.
>Just for the record, the other mismatches are at DYS 388 (A=11, B=12) and
>DYS 458 (A=16, B=18). They match on all the other markers.
Now adding in the above DYS388 an 458 we have another 3 steps for a total of 7
The odds of two men being related with 6 mutations in 15 births is 1 in 100,000
Thus the Y-DNA results to not support the proposition that these men as have
a common male ancestor in the last 400 years or so.
Note that two unrelated men will typically have a mismatch of 5 to 10 steps.
Also note I did not say it cannot happen - just that the odds against it is
100,000 to 1
It is much more likely there is an undocumented adoption or other event.