Posted by Larry Hoover on November 29, 2005, at 16:18:44
In reply to Re: This was a rhetorical question everyone » SLS, posted by alexandra_k on November 14, 2005, at 12:29:09
> >... twin studies have shown that alcoholism is heritable.
>
> Twin studies have shown that addictive behaviour HAS A HERITABLE COMPONANT. There is a difference. It might sound picky but it is the difference between having the heritable componant thus HAVING to display the addictive behaviour, versus having the heritable componant thus BEING MORE LIKELY to display the addictive behaviour. If you are the person with that heritable componant... That is a very big difference indeed...I wonder what the heritability estimate is for being anal?
I can't help it. Larry Hoover's phenotype, 99% anal. (I'm not perfect.)(But that's anal to say that.) Being me is hard. ;-)
Just to interject something about a subject that means a lot to me....
Heritability is a population parameter. It is not, and cannot be, applied to individuals.
http://psych.colorado.edu/hgss/hgssapplets/heritability/heritability.intro.html
One of the reasons it is an estimate is that there are so many confounds. It is a fuzzy term, and cannot be definitively applied, even to populations. It is a term never applied to individuals.
Genes are not destiny, even if the heritability estimate of a phenotype is well defined.
Identical twins could not be discordant, were it otherwise. Heritability from twin studies could not be estimated, otherwise.
Confounds in twin studies (separated at birth) include: individual vs. separate placentas; if separate placentas, proportional exposure to maternal blood supply; if one placenta, differential in blood flow to each fetus (both bear on oxygenation, nutrient supply, and bolus exposure to toxicants, and can be inferred by birth weight differentials); post-differentiation genetic drift (once the fetuses (feti?) separate, their genes can mutate); differential in complications in utero and at birth.
Any differences in these or other undefined parameters lead to under-estimation of heritability.
My brother is the proud father of identical twin daughters. One has Down's Syndrome. It is believed that the mutation occurred roughly two days after differentiation of the blastocysts/embryos. The Down's child is called a chimera, as she has cells both with and without the mutation.
I now return you to your most interesting debate.
Lar
poster:Larry Hoover
thread:575263
URL: http://www.dr-bob.org/babble/subs/20051106/msgs/583400.html