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.

ok? ok.

now, my question is: how related are these people, genetically?

a guy and his fbd share 1/8th or 12.5% of their autosomal dna (probability says) — and that’s it. they don’t share a y-chromosome like the guy does with his fbs. just 1/8th of their autosomal dna.

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].)