not very much, genetic relatedness-wise. i think it might be pretty much like marrying any random person from your ethnic group (i’m not sure — maybe someone out there knows?).
as we’ve seen, once christianity hit europe, certain bans on whom you could marry were instituted. by the sixth century, you could no longer marry your second cousin (if you were going to be a good christian, that is). later that was pushed back to the fourth cousin; and in the eleventh century, it was your sixth cousin. the regulations dropped back to fourth cousin again in the thirteenth century. those are big changes compared to pre-christian days when marrying your first cousin was prolly not uncommon.
so, how do these mating patterns affect the degree of genetic relatedness in a society?
well, looking at the standard coefficients of inbreeding for cousins, i worked out that the coefficient of inbreeding for fourth cousins is 0.000977 (that’s rounded to the sixth decimal place), and sixth cousins is 0.000015. first cousins, btw, is 0.0625.
using falconer’s coancestries recurrence equation, we can calculate, for instance, how inbred an individual is at the end of three generations of the same sort of inbreeding (e.g. first cousin marriage each generation).
if you have three successive generations of first cousins inbreeding (barring any other sort of inbreeding), the math looks like this:
Ft = 0.0625 (1 + 3Ft-1 + Ft-2)
Ft = 0.0625 (1 + 0.1875 + 0.0625) = 0.078125
so, at the end of the third generation, we get individuals with a quite higher coefficient of inbreeding than their cousin-grandparents.
fourth cousins, not so inbred:
Ft = 0.000977 (1 + 3Ft-1 + Ft-2)
Ft = 0.000977 (1 + 0.002931 + 0.000977) = 0.000981 [rounded]
seventh cousins … well, like i said, for all i know this level of inbreeding might be like that between two strangers from the same population:
Ft = 0.000015 (1 + 3Ft-1 + Ft-2)
Ft = 0.000015 (1 + 0.000045 + 0.000015) = 0.000015 [rounded]
imagine having several hundred years of such strong outbreeding. that’s some really loose genetic ties, afaics. imagine several hundred years of strong inbreeding!
Ft = 0.0635 (1 + 3Ft-1 + Ft-2)
Ft = 0.0635 (1 + 0.1905 + 0.0635) = 0.079629
yup. more inbred than regular first-cousin cousin marriage!
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