86 ( H \r 1 1 k 6 



can be investigated bj using morphological traits. Although phenogenetics often starts 

 out as a study of the developmental genetics of morphology, the final morphological 

 outcome which usualh is a pleiotropic one — is often found to be based upon earlier 

 morphological changes which arc. in turn, preceded by still earlier-occurring physio- 

 logical changes. Consequently, the developmental genetics ol morphological features 

 is based upon gene-caused physiological changes and leads to a study of physiological 

 genetics. 



Physiological genetics reveals that the physiological effect o\ the genotype is some- 

 times intracellular and sometimes intercellular. The gene-based action that certain 

 cells have on others can involve a general or localized control of growth rates and dif- 

 ferentiation. This action can occur nearby, via induction; or at a distance, by means 

 of a general nutritive effect, by hormones, and probably by nerve impulses and muscular 

 contractions. Gene changes can modify the competence of a tissue. 



Comprehension of physiological genetics must, in turn, ultimately involve a detailed 

 understanding of how genes influence metabolism, and since metabolism involves the 

 study of physical and chemical reactions, phenogenetics must ultimately be described 

 in biophysical and biochemical terms. The phenogenetic study of the gene causing 

 sickle cell anemia proceeded from morphology to physiology to biochemistry. 



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