GENEALOGY-DNA-L ArchivesArchiver > GENEALOGY-DNA > 2010-03 > 1268959006
Subject: Re: [DNA] Expected NPE's Over 800-1000 Years
Date: Thu, 18 Mar 2010 20:36:46 EDT
The probability that a living male will carry the Y-signature of his
surname’s progenitor from n generations ago is (1-p)n, where p is the fractional
probability of a non-paternal event at each generation. So first you have
to decide what rate of NPEs per generation you think is reasonable and
then you have to decide how many years constitute a generation. I have read
where some experts think 2% to 3% per generation is a reasonable estimate of
NPEs and many experts use 25 to 30 years to calculate a generation.
If you use 30 years per generation, 800 years equals 26.67 generations and
1000 years equals 33.33 generations. If you use 2% per generation as the
average rate of NPEs, this suggests at the 25 generations 60% of the
descendants of the progenitor will carry his Y-DNA signature and 40% will not.
At the end of 35 generations, 49% of the descendants of the progenitor will
carry his Y-DNA signature, and 51% will not.
Hope this helps.
Thank you--I believe that is the calculation I recall seeing. Now that
I've seen it, I doubt I could ever do more than guess how many new Britton
families may have been spawned by X number of Britton families which existed
in England c 1200; however, I wouldn't be surprised if half or more of
those in my project are the result of NPE's somewhere along the line. It
seems to me that the greater the number of close matches a family has, the
greater the probability that it no longer bears its original surname. If
that's true, then my family might just be an original, because we don't yet
have any close matches whereas the others do.