CHAPTER 19 



Genetic Factors 



The factors discussed thus far are effective chiefly through the environ- 

 ment of the plant, either its external surroundings or its inner physiologi- 

 cal processes, which are open to relatively simple analysis. It is obvious, 

 however, that these factors alone are not enough to explain all mor- 

 phogenetic phenomena. There are also inborn differences, rooted in the 

 specific constitution of each individual organism, which powerfully affect 

 what it is and does. These differences are inherited, and it has been the 

 great service of genetics to biology since the turn of the century to show 

 that their physical basis is primarily in the genes, located in the chromo- 

 somes of the nucleus. 



GENES 



The various environmental factors exert their effects against this 

 specific genetic background, the entire system of genes in the plant. 

 Genes are sometimes thought of as though these bodies, known to be 

 independent in inheritance to a certain degree, were also independent 

 in development. This evidently is not true, however, for in their control 

 of growth and differentiation the action of all the genes must be closely 

 coordinated, in space and time, if an organism is to be produced. How 

 these distinct entities are thus so precisely correlated in their action is a 

 major problem for both genetics and morphogenesis. It is also clear that 

 a gene does not produce its effect by determining a precise series of steps 

 leading to the development of a specific trait, for the same genotype 

 may have a very different effect if the environment is different. A gene 

 simply determines a specific response to a specific environment. The 

 genetic constitution that distinguishes a tall plant from a short one, for 

 example, will not produce this difference unless the conditions of tem- 

 perature, moisture, and soil fertility are such as to make vigorous growth 

 possible. 



The problem of gene action, of how a gene or group of genes produces 

 its effects, is now one of the central concerns of genetics and is being 



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