Development 59 



How is this buffering accomplished? Using the model of Lerner 

 let us assume that gene A produces substance o, which is modified 

 by the action oi b {a product of gene B ) into r, and that r interacts 

 with c (produced by gene C) to give substance t. 



( After Lerner, Genetic Homeostasis, 

 Wiley, New York, 1954. ) 



Now if mutation removes B or if the environment lacks the substrate 

 from which b can be manufactured, then a will accumulate. A svs- 

 tem in which high levels of a interact with d (product of gene D) 

 to make s, which in turn can be transformed by c into t, is a buffered 

 system. 



< ^^ ( After Lerner, Genetic Homeostasis, 



C — > C ^ Wiley, New York, 1954. ) 



This is a true feedback system, since the exact course leading to 

 normal character expression is determined by the "information" that 

 the organism has with respect to the level of a. This does not mean 

 that all buffering is genie, however. 



It is common practice to draw a sharp line between genotype and 

 phenotype. As a pedagogic device, this is useful for emphasizing the 

 relative permanence and continuity of the genetic information, and, 

 although an oversimplification, it has led to considerable progress 

 in the science of genetics. However, it has also led to the impression 

 that the genotype is somehow the basic entity and that the pheno- 

 type is merely a crude reflection of the genotype (the image of 

 which has been distorted by the environment). One might well 

 wonder why selection has not done away with the phenotype alto- 

 gether, permitting the genotype to evolve unsullied. The answer is, 

 of course, that what can be separated in textbooks or in theory 

 cannot be separated in living organisms. If the genetic material 

 were dissected from a fruit fly, one would obtain a long meaningless 

 string of nucleotides, itself an aspect of the "phenotype." It is clear 

 that at this level of study the distinction between genotype and 

 phenotype is meaningless. The genetic information becomes mean- 



