geneticists will tell you that the percentage of their genomes that two first-cousins will probably share is 6.25% (standard deviation ±2.4% — i’ve been wondering that, too!). but that’s in a randomly mating population. i’ve been wondering what happens in a population where cousin-marriage is the norm and has occurred generation after generation — like in saudi arabia or pakistan. what percentage of their genomes are those kinds of first-cousins likely to share?
well, now i have an answer. (it might not be the answer ’cause these researchers were looking only at families with congenital disorders, so … you know … ascertainment bias and all that.)
in “Quantification of Homozygosity in Consanguineous Individuals with Autosomal Recessive Disease,” woods, et. aaaaallll. (there’s a LOT of them), found that, in populations with regular, long-term cousin-marriage over many generations — in saudi arabians and pakistanis, as a matter of fact — first-cousins shared, on average, 11% of their genomes in common. that’s approaching twice as much as first-cousins in a randomly mating population.
“In conclusion, we found that the amount of homozygosity is greater than expected (11% observed vs. 6% expected) in individuals with autosomal recessive disease whose parents are first cousins and who come from communities that frequently practice consanguineous marriage. First-cousin offspring had as much homozygosity as would have been expected for double–first cousin offspring.”
so, first-cousin marriages in places like pakistan and saudi arabia (and afghanistan, etc., etc.) are, on average, more like double-first-cousin marriages in places where mating is more random, like large segments of the u.s., europe and the west in general. maybe.
here, once again, are mr. light-blue and ms. pink and their offspring, mr. dark-blue, to illustrate the differences for you (click on images for a LARGER view — should open in new tab/window):
mr. light-blue and ms. pink randomly mating:
here are the proud parents with their offspring, mr. dark-blue, who inherited
the family fortune half of his genome from his father, and the other half from his mother:
here are mr. light-blue and ms. pink again, only this time they are first-cousins and share (prolly) 6.25% of their genomes in common (the bits that overlap):
here they are with blue, jr., who has still inherited half of his genome from each of his parents, but he’s got two identical (by descent) copies of some of his genome — he is less unique than mr. dark-blue above (see how he’s narrower?):
here are mr. light-blue and ms. pink one last time — they are first-cousins once again, but they come from a society where cousin marriage is the norm and are the descendants of a line of cousin marriages. the “overlap” of their genomes (11%) is larger than the first cousins above:
and here’s the final incarnation mr. dark-blue who is even less unique than mr. dark blue ii because he inherited even greater numbers of genes that are identical by descent from his parents:
(note: comments do not require an email. mr. pink.)