# fbd marriage – genetic relatedness amongst nuclear family members

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.

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].)

1. I can’t help with the math but it’s interesting.

one way or another this kind of thing will affect something or have affected something in the past imo

2. @g.w. – “this kind of thing will affect something”

well, i think that, for one thing, it must affect the strength of feelings people have towards their various kin (and non-kin, for that matter).

if, for instance, one father is more related (genetically) to his sons and/or daughters than another father, you’d think — if our feeling towards our kin are really governed by the “rules” of inclusive fitness, selfish genes, and all that (which they seem to be) — that the more closely related father would feel stronger towards his kids, in whatever ways that fathers feel towards their kids, than the not-so-related one.

i think this should apply in all sorts of directions.

(did that make any sense at all?)

3. Not to interfere with your 101 notes but I have to ask: What’s your beef with capital letters? This stuff is difficult to read. What’s next – dispense away all punctuation?

4. @nanonymous – “What’s your beef with capital letters?”

no beef. just a long-time habit. you’re not suggesting that this old dog try to learn a new trick, now, are you? (~_^)

5. “i think this should apply in all sorts of directions. (did that make any sense at all?)”

Yes totally.

If endogamy acts in a highly beneficial way among small groups but also as a kind of centripetal force working against larger scale co-operation then it seems to me marriage systems could have been quite critical in human development generally. A change in a system away from the most endogamous possible (past the incest block) to a slightly less endogamous one could have lowered the co-operation threshold to the point where two adjacent clans formed a tribe.

(It’s similar to the sort of idea of a woman having a son with one clan and then being captured by an adjacent enemy clan and having a son with them and then eventually the two half-brothers meet up in a battle over the nearby watering hole and don’t want to kill each other.)

Another thought would be what if a specific form had been adopted for a specific benefit i.e the benefits in small-scale violence from having all the males as closely related as possible, but at the same time that benefit made it hard to break out of that system so it became a dead end. Armies have always consciously striven to recreate that “Band of Brothers” effect but maybe across a whole society its usefulness is restricted to small nomadic clans.

(The idea provides an interesting alternative take as to why scientific progress in the middle east died out in the centuries after the Arab conquest. The marriage system changed, endogamy increased too much, and the level of co-operation dropped.)

The other thought i had was how this fits into mythology. It’s been a long time since i studied it but IIRC it’s full of this kind of stuff to do with relatedness.

6. @gw – “The idea provides an interesting alternative take as to why scientific progress in the middle east died out in the centuries after the Arab conquest. The marriage system changed, endogamy increased too much, and the level of co-operation dropped.”