RE: Who knew genes could get mean?

From: Gatherer, D. (Derek) (D.Gatherer@organon.nhe.akzonobel.nl)
Date: Thu Dec 21 2000 - 13:45:32 GMT

  • Next message: Vincent Campbell: "RE: Who knew genes could get mean?"

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    From: "Gatherer, D. (Derek)" <D.Gatherer@organon.nhe.akzonobel.nl>
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    Subject: RE: Who knew genes could get mean?
    Date: Thu, 21 Dec 2000 14:45:32 +0100
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    Vincent:
    For example, I heard once that in the UK at least, you wouldn't
    need to go back more than 150 years on average to find a common ancestor
    between any two randomly selected people.

    Derek:
    I think it's a little longer than that, about 22 or so generations. But the
    variance will be high, as its clear that most preindustrial revolution
    communities were fairly isolated. These predictions are theoretical, and
    based mostly on the calculation of the probability that 2 people don't
    overlap in a set of 2powerN ancestors going back N generations. So where N
    = 22, you have just over 4 miilion ancestors. If the generation time is 25
    years on average, you're back to about 1450 when the population of mainland
    Britain was about 4 or 5 million. So every possible ancestor is accounted
    for. However, inbreeding within small villages, for instance, would mean
    that the number of ancestors is far short of 2powerN, and one would have to
    go back quite a bit further in order to account for all possible ancestors.
    Relatedness can only really be tested empirically for those who have full
    pedigrees, eg royalty. They do seem to be all related eg. see
    http://www.dcs.hull.ac.uk/public/genealogy/royal/catalog.html, but they are
    clearly a special case because of all the inbreeding. In genetic terms,
    they might represent a sort of endogamous village, albeit one with rather
    plusher mudhuts than most. So the claim often made eg see
    http://www.dcs.hull.ac.uk/public/genealogy/royal/FAQ.html#Q4,

    "It is not uncommon for people of British ancestry to descend from some
    Royal or other. It has been said by some that all English descent from
    Edward I and all Scots from Malcolm III. Therefore there is no particular
    cachet in having any royal ancestors, but proving it is another matter
    entirely!"

    is a bit difficult to assess. If you believe the straight 2powerN argument,
    then Edward I is a common ancestor of all Englishmen as he lived a good 6
    generations before the 1450 threshold, but on the other hand if your
    ancestors were entirely confined to a small village in the middle of
    nowhere, then it would depend entirely on the rate at which exogenous genes
    arrived in the population.

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