Re: A new topic.

From: Malcolm (malcolm_at_55bank.freeserve.co.uk)
Date: 12/19/04

  • Next message: William Morse: "Re: A new topic." 
    Date: Sun, 19 Dec 2004 05:37:03 +0000 (UTC)

    <innominatetwice@yahoo.com> wrote
    >
    > Okay, I have a new question: When a Black person and a White person
    > have a child, why is it that (in my opinion) the characteristics of the
    > child appear as an "averaging" of the parents traits? For instance,
    > their skin color, body shape and so on.
    >
    That is because skin colour is determined by half a dozen genes. It does
    occasionally happen that a mixed race family has one very light-skinned and
    one very dark-skinned child, but usually what you get is an intermediate.
    You would also get the same effect, in the first generation, if the genes
    were co-dominant, i.e. neither recessive but both contributing equally.
    However if two mixed raced children were to marry, the children would then
    be 25% white, 25% black and 50% mixed. This doesn't happen, so we know that
    co-dominance of a single gene is not the explanation.
    >
    > What I want to know is: how is it possible for the two sets of genes to
    > work together and yet interfere with each others results? And to do so
    > in a way that seems predictable and regulated.
    >
    Absolutely. This is technically known as epistasis. When you have more than
    a few genes involved, epistatic effects are very difficult to tease out. In
    fact there isn't a single case of a multifactorial trait whose genetics are
    fully explained. For instance, we know that two genes, BRCA1 and BRCA2,
    predispose to breast cancer, but not all women with the cancer-disposing
    alleles go on to develop the disease. It may be that the reason is that some
    women are lucky enough to avoid environmental triggers, or it may be that
    they have other genes that mask the effect. Allergic people, with
    over-active immune systems, tend to be protected against cancer to some
    extent, for instance, so if a woman has BRCA1 then she may develop cancer
    cells, which are then destroyed by her immune system, if she is also
    allergic. What we don't know yet is exactly which genes are involved, and
    how they interact, so it is not possible examine a woman's DNA and say "you
    will develop breast cancer at the age of fifty unless you have surgery to
    remove the breast at the age of 49".

  • Next message: William Morse: "Re: A new topic."

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