Re: (newbie) Basic question

in article e91phr$1vdo$1@xxxxxxxxxxxxxxxxxxx, Perplexed in Peoria at
jimmenegay@xxxxxxxxxxxxx wrote on 7/11/06 8:16 PM:


"Guy Hoelzer" <hoelzer@xxxxxxx> wrote in message
news:e8up49$gra$1@xxxxxxxxxxxxxxxxxxxxxx
in article e8rvep$24jt$1@xxxxxxxxxxxxxxxxxxx, john edser at
edser@xxxxxxxxxxxxxx wrote on 7/9/06 3:20 PM:
Guy Hoelzer <hoelzer@xxxxxxx>
... If a trait is under the influence of many genes it can
be genetically controlled without having almost any heritability.

JE:-
Please provide an empirical example.

GH:_
Number of limbs in humans.

Which raises the question of just what it means for a trait to be
"genetically controlled." (To say nothing about what it means for
a trait to be "heritable".)

Indeed. This distinction is frequently confused. Traits can be under strong
genetic control, yet have little heritability. They can also be under
strong environmental control and have high heritability. Of course, this is
a very important distinction to keep in mind with regard to evolution in
general, and natural selection in particular. Heritability is the only
thing that matters in evolution. If we are concerned with the evolution of
organismal traits, then genetics is only relevant to the extent that they
influence the heritabilities of those traits. We have focused almost
exclusively on population genetic models of organismal evolution over the
past 75 years, while ignoring the role of environment in heritability. We
often take the genetic models as metaphors that can represent the
environmental component of heritability, but they also assume genetic
mechanisms that are not like those of environmental inheritance. In my
view, theoretical developments in the area of environmental inheritance and
evolution will be a profitable field in the future.

The argument might be made that during the period when thalidomide
was being prescribed in Europe, the limb count in humans was
environmentally controlled. Or, after visiting a VA hospital, you
might make that argument today.

Indeed again. I see all traits as being influenced by both genes and
environment, although the influence from each source can vary dramatically
among traits. The novel change in the environment that occurred with the
introduction of thalidamide resulted in a big change in the relative
influence of genes and environment in the development of human limbs.

I might have offered John the 'trait' of being a blue-eyed PTC taster
with dry earwax and blood type AB negative. String together enough
simple genetically controlled traits with moderate heritabilities
and you get a compound trait of vanishing heritability.

I agree. That is a better approach, especially when you think of traits the
way I do. IMHO, traits are not real things. They are arbitrarily bounded
qualities, or sets of qualities, of phenotypes. So, in my view, the set of
things you describe taken together are a perfectly good single trait.

The difficulty with giving a natural _empirical_ example is that most
complex-genetics natural traits are not well enough understood to be
sure that they are mostly genetically controlled.

True enough.

Cheers,

Guy


.

Relevant Pages

  • Re: (newbie) Basic question
    ... be genetically controlled without having almost any heritability. ... Number of limbs in humans. ... I wonder if you're confusing genetic control of traits with genetic ...
    (sci.bio.evolution)
  • Re: To whom "Romeo and Juliet" makes no sense?
    ... Heritability is defined as the proportion of variance in a trait which ... monozygotic twins would be 1.00 if genetics alone accounted for ... positive parenting outcomes are linked more ... and many books in the home environment. ...
    (rec.arts.sf.written)
  • Re: Heritability of fitness
    ... >> the rate at which information 'about the environment' is accumulated ... >> in the genome and the heritability of total fitness. ... >> should be very close to zero. ...
    (sci.bio.evolution)
  • Re: Heritability of fitness
    ... > in the genome and the heritability of total fitness. ... > close to zero. ... > enough to measure - at least over timescales in which the new environment ...
    (sci.bio.evolution)
  • Re: Cortical development and IQ
    ... heritability does not equate with genetic determinism. ... Race, Intelligence, and the Brain: ... their children will grow up with a better environment (created through ... "Intelligence, Race, and Genetics: ...
    (sci.bio.evolution)