>Hi
>To add to the discussion on so called genetic coefficients.  We at the SA
>Sugar Association have spent some effort in trying to formulate "genetic
>coefficients" (some plant breeders really get excited when this term is
>used because they use the term for very different parameters and feel that
>it creates confusion) for sugarcane that capture genetic characteristics
>and by implication exclude environmental effects.  Preliminary studies
>showed that very few of the coefficients that we have investigated seem
>stable across environments.  This implies that (1) the data  is too
>variable and that measurements are not adequate or (2) the coefficients do
>not truly capture the gene action as it manifests in phenotype.  Many of
>these coefficients refer to phenotypic events such as thermal time to leaf
>emergence or maximum leaf size etc. and are far removed from the actual
>genetic control of processes in the plant.  They are convenient in many
>instances but could be quite misleading.  I agree with Ken and believe
>that these coefficients should be viewed as attempts at understanding the
>true genetic control of growth and development.  We could learn a lot from
>how these coefficients vary between environments.  It is interesting to
>see how different crops are approached e.g. soybeans (Hoogenboom & White,
>Agron.J. 95:82-89) has succeeded in very closely capturing genetic control
>of development by formulating coefficients in terms of gene presence or
>absence.  To do that with a crop like sugarcane would be extremely
>difficult because of its polyploidy -  we therefore have to formulate
>coeff's in terms that are a few levels removed from the actual control by
>genes.
>
>In summary I think that as we try and unravel genetic and environmental
>control of plant processes, the models of these processes and the
>parameters that we use to represent genetic traits will improve to more
>closely represent reality.  Where coefficients do exhibit variance across
>environments, that reflects lack of knowledge in some cases, or - a
>realization that further refinement to achieve closer representation of
>reality will not achieve significant improvement in model applications.
>
>Our preliminary efforts are documented in
>Zhou, M., Singels, A. and Savage, M.J., 2003. Physiological parameters for
>modelling differences in canopy development between sugarcane cultivars.
>Proc S Afr Sug Technol Ass 77:610-621
>Donaldson, R.A., Redshaw, K.A. and  Singels, A., 2003.  In search of
>parameters to model cultivar specific canopy development. Proc S Afr Sug
>Technol Ass 77:605-609
>
>     Thanks for the opportunity to be part of this stimulating dicussion.
>
>Abraham Singels
>SA Sugar Association Experiment Station
>Mount Edgecombe
>