On 14/02/15 15:41, Charlie Hoffpauir wrote:
> What I'm looking for is some software that will perform this
> calculation, instead of laborously doing the math by hand. [...]
> (actual relationships for me and a cousin)
> 1. Fifth cousin once removed (common ancestor: Thomas Hoffpauir &
> Marie Charlotte Périllard)
> 2. Fourth cousin twice removed (common ancestor: Thomas Hoffpauir &
> Marie Charlotte Périllard)
> 3. Fifth cousin once removed (common ancestor: Edward Foreman II &
> Nancy Anna Perry)
> 4. Fifth cousin once removed (common ancestor: Edward Foreman II &
> Nancy Anna Perry)
> 5. Fourth cousin twice removed (common ancestor: Joseph Ephraim
> Foreman & Mary Elizabeth Brown)
> 6. Fifth cousin twice removed (common ancestor: Edward Foreman)
> 7. Sixth cousin once removed (common ancestor: Edward Foreman)
I assume this is so you can interpret the results of autosomal DNA test?
An nth cousin m times removed contributes 1/2^(2n+m-1) to the
coefficient, assuming the children of the most recent common ancestor
are full siblings but not identical twins. Those seven relationships
give: (1) 1/1024, (2) 1/512, (3) 1/1024, (4) 1/1024, (5) 1/512, (6)
1/2048, and (7) 1/4096 -- assuming the latter two have a pair of common
ancestors. (Halve them if they're only half fifth cousins twice
removed, etc.) These are additive, so the coefficient is 31/4096 or
about 0.8%. As I understand it, that's a little above the limit of what
can be normally be detected. That limit comes from two fairly
fundamental factors.
First, it's very unlikely that you have inherited genetic material from
all of their four-great grandparents. I'm not considering non-paternity
events here: rather the process of DNA replication means its more likely
than not that a given gene will be copied from the same parent as the
neighbouring gene. The norm is long sections copied from a given
ancestor, and before long some ancestors are no longer represented in
your genetic make-up. If either you or your cousin have not inherited
material from the common ancestor, you're out of luck.
Secondly, this degree of relationship is close to the background level
for many cultures. Any two people from the same community will have a
great many common ancestors if you go back far enough. Over twenty
generations they'll typically thousands of common ancestors once
pedigree collapse has been considered (i.e. if you count ancestors
multiple times, once per descent); if you're from an isolated community,
whether geographically isolated or culturally, that number will be
larger. I think these unknown common descents within a community
results in a background level somewhere around 0.1%, to within an order
of magnitude.
These are not principally technological constraints, so are unlikely to
improve if you wait ten years.
Richard