’cause you can never have too much of a good thing, right? (~_^)

double first-cousins are just that — first-cousins twice over:

“‘Double first cousins’ arise when two siblings of one family reproduce with two siblings of another family. The resulting children are related to each other through both parents’ families. Double first cousins share both sets of grandparents in common and have double the degree of consanguinity of ordinary first cousins. Genetically, they are as related as half-siblings.”

double first-cousin (d1c) marriage looks (or can look) like this:

in the parts of the world where father’s brother’s daughter (fbd) marriage is practiced, d1c happens at not a small frequency. in afghanistan, for example, 6.9% of consanguineous marriages are to double-first cousins (46.2% of marriages are consanguineous).

d1c marriage obviously changes the degree of relatedness within the d1c family, compared to other cousin-mating families that is. broadly speaking, double-first cousins are like half-siblings in terms of genetic relatedness.

if you use my system for calculating new-and-improved coefficients of relationship/relatedness, which is based on the differential inheritance of the x- and y-chromosomes, then the relatedness of two brothers from a father’s brother’s daughter double first-cousin mating (fbd d1c mating) looks like this (if i’ve done the maths right!**):

B-B (FBD D1C marriage)
**a man and his FBD share 1/8 of their autosomal dna; a woman and her MZS share 1/8 of their autosomal dna + 1/4 of an x-chromosome + another 1/8 of an x-chromsome.
**therefore, two brothers from an FBD D1C marriage share: 1/2 autosome + 1/16 autosome + 1/16 autosome + 1/2 x-chromosome + 1/8 x-chromosome + 1/16 x-chromosome + y-chromos
(96.42% x 0.5) + (96.42% x 0.0625) + (96.42% x 0.0625) + (2.60% x 0.5) + (2.60% x 0.125) + (2.60% x 0.0625) + 0.99% = 0.5701

so, an fbd d1c mating raises the genetic relatedness of two brothers to the same level as that of two brothers from an mzd marriage:

what would be interesting to know is if fbd d1c marriage was more prevelant in the past when transport wasn’t so good and people prolly didn’t marry over such long distances. in other words, there might’ve been more cousin marraige, including double first-cousin marriage, in general in the past.

**have i got 1/16 autosomal dna too much there with the two brothers (B-B)?

(note: comments do not require an email. i’m my own grandpa!)

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