ok. so here’s what father’s brother’s daughter (fbd) marriage looks like:
the first thing to notice is that if you marry your fbd (or your fbs if you’re a chick) is that your children are now also your cousins.
yes. ’cause, if your cousins’ children are your cousins (e.g. your first-cousins’ children are your first-cousins once removed), then if you marry your first-cousin, then your kids are your first-cousins once removed.
now, my question is: how related are these people, genetically?
so, now we can work how how related these nuclear family members are.
normally (without any inbreeding) this is what the genetic relatedness looks like:
F–s = 1/2 autosome + y-chromosome
F–d = 1/2 autosome + 1 x-chromsome
M–s/d = 1/2 autosome + 1 x-chromsome
with the inbreeding of two first-cousins, though, we have to add in that 1/8th that is shared between them. and, if i have this right (do i have this right, r.a.? anybody?), the new calculation should look like this:
F–s = 1/2 autosome + 1/16th autosome + y-chromosome
F–d = 1/2 autosome + 1/16th autosome + 1 x-chromosome
M–s/d = 1/2 autosome + 1/16th autosome + 1 x-chromsome
the coefficients of relatedness (rounded to four decimal points) i get then are:
F–s/s–F = 0.5523
F–d = 0.5684
d–F = 0.5593
M–s/d = 0.5593
s–M = 0.5684
d–M = 0.5593
in other words, the people above (probably) share a lot more genes than this nuclear family (below) that is not inbred:
F–s/s–F = 0.4920
F–d = 0.5081
d–F = 0.5000
M–s/d = 0.5000
s–M = 0.5081
d–M = 0.5000
that’s all for now!
(note: comments do not require an email. or a slide-rule [probably].)