It is not true that if something is influenced by genetics is is "heritable by definition." That is because all traits of organisms are "influenced by genetics" - genetics an the environment are all there are to influence any trait. Thus the trait of speaking English is influenced by genetics, and is partially explained by a biologically based capacity to learn languages, a capacity which is itself heritable, but the trait of speaking English is decidedly not heritable.
Further, the "central dogma" of molecular biology is that DNA is transcribed to RNA, then to protein, never the reverse direction, at least from protein to DNA/RNA (reverse transcription, RNA-->DNA is not uncommon) -- this is a rejection of the heritability of environmentally caused changes in (genetically based) traits resulting from the actions of the proteins whose behavior is directed by genes (DNA sequences). This is a modern day rejection of Lamarckianism, the doctrine of the heritability of acquired characteristics. But obviously the behavior of proteins and the traits they cause are "influenced by genetics. We speak of traits as heritable, but strictly what is heritable in the information in the DNA, not the traits as manifested in the phenotype and influenced by the environment.
For a moderately technical discussion of heritability ( a very vexed concept often misunderstood) see:
http://plato.stanford.edu/entries/heredity/
The non-numerate can skip the equations and just read the prose.
--- Doug Henwood <dhenwood at panix.com> wrote:
>
> On Nov 22, 2007, at 1:37 PM, Miles Jackson wrote:
>
> > I'm not sure if I understand the distinction. If
> something is
> > influenced by genetics, then it is heritable by
> definition. (Genetic
> > transmission is the basis of heritability.)
>
> Of course; I know the basics of genetics. But there
> is enormous
> variation between parents and children, and from one
> sibling to
> another. It's a big leap from comparing twins to
> comparing other
> kinds of relatives, even first-degree ones.
>
> > Here's the chain of reasoning: if genetics has
> nothing to do with IQ
> > performance, then the correlation in IQ scores
> between biological
> > parents and the children they have no contact with
> should be zero.
> > The
> > observed correlation is around .20. Yes, I agree
> that this is a
> > relatively weak predictive relationship; however,
> it does exist. This
> > does provide some evidence for genetic influence.
>
> Ok, .20. That was my guess. That's well above 0, but
> it still leaves
> 80% of the story to be explained. For popular
> consumption, it's
> barely worth talking about.
>
> This genetics shit is getting sinister in pop
> culture. The NYT's Amy
> Harmon has now gotten her genome done and she's
> looking to explain
> her life with it. This level of determinism seems
> almost un-American.
>
> Doug
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