endogamy and genetic relatedness

this is just a short note — a little food for thought.

the following quote comes from catherine linley day’s FABULOUS ph.d. thesis, “Marriage Patterns in Two Wiltshire Parishes 1754-1914: Geographical Mobility, Consanguinity and Illegitimacy” [opens pdf]:

“In a theoretical isolated population of 500 people, after six generations all potential marriage partners would have been related to each other as 3rd cousins or closer (Fox 1967).”

the fox 1967 reference is to robin fox’s Kinship & Marriage: An Anthropological Perspective. i haven’t looked through fox’s book, yet, to see where linley day got this from.

in any case, it’s interesting to see how quickly endogamous mating patterns can lead to everyone in the population being quite related to one another (like in iceland or ashkenazi jews), genetically speaking. of course, no human population is totally isolated (right??), so you won’t find this exact scenario out in nature. but it’s interesting — and important — to keep in mind.

update 7/10 – calculating the inbreeding coefficient (see comments below):

(note: comments do not require an email. extended family.)



  1. For a little perspective, how many generations do you have to go back before the number of ancestors exceeds the world population? A thousand years as I recall. Maybe I am wrong.


  2. Follow up: I see that 2 raised to the 50th power = 1125899907000000, which is way more than enough. 5 generations to a century = 10 centuries.


  3. @luke – “So if we are all cousins, what is the most distant cousin it is possible to be?”

    good question! i dunno. -?-

    if you look at this wikipedia page, you’ll see the inbreeding coefficients for various family members (in randomly mating societies). the inbreeding coefficient for first cousins=0.0625, second cousins=0.015625, and third cousins=0.00390625.

    you and your clone would have an inbreeding coefficient of 1. you and someone to whom you are totally and utterly unrelated would have an inbreeding coefficient of 0. so, the most distant cousin it is possible to be would be the one with whom you’d have an inbreeding coefficient as close to 0 as possible without reaching 0.

    i’ve added a formula for calculating the inbreeding coefficient to the bottom of the post above. p is the number of generations back to the common ancestor in the paternal line and m is the number of generations back to the common ancestor through the maternal line. you could try to work out the most distant cousin that way. (^_^)

    the short answer is prolly: very, very distant — and you’d hardly be related to that person at all.


  4. It’s said that every single person of European origin is a descendant of Charlemagne. With Charles’ first children born around 770, and a generation time of 25 years, that’s about 50 generations.

    A rough estimate of the population in 800 AD is 32 million. 25 generations back, there are 32 million spots on your family tree. That gets you *halfway* to Charlemagne.

    Incidentally, are there good ways to estimate consanguinuity from a 23andme-type genotype? In my case, the problem is a little harder – my parents are from opposite ends of the Mediterranean, so are probably only very distantly related. But it would be interesting to know how inbred their ancestors were.


  5. @anthony – “Incidentally, are there good ways to estimate consanguinuity from a 23andme-type genotype?”

    not that i know of.

    you could use a program called PLINK to check for runs of homozygosity (roh) in your genome, and from that you could sorta infer how inbred you are, i.e. if you found you had a LOT of short roh, you might be able to guess that you’re pretty inbred (prolly not, though, if your parents are from different ends of the med, like you say).

    if you had 23andme data for each of your parents, you could run those through PLINK to see how inbred your parents are. (^_^) (if you figure out how to use PLINK, lemme know — i haven’t tried it yet!) you might be able to do other cool things with PLINK, too, i dunno — i haven’t read all about.


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