PHYSIOLOGICAL GENETICS 



I. INTRODUCTION 



Genetics is the study of heredity. As the causation of all 

 hereditary traits must be found in the fertilized egg (or its 

 physiological equivalent in protista and lower plants), it is the 

 aim of genetics to link the agents within the germ cells with the 



hereditary traits observed in the fully developed individual. 

 There 1 are two main aspects of this problem: first, the unknown 

 agents in the germ cells have to be identified; their mode of 

 transmission in all its phase- has to be ascertained; and the facts 

 found have to be brought in line with the results of genetic 

 experimentation. As is generally known, the study of these 

 problems has accumulated the bulk of facts usually considered 

 as genetics proper, culminating in the generally accepted theory 

 of the gene as the hereditary unit and the chromosomes as its 

 seat. This side of the problem of heredity may be called the 

 problem of the mechanism of heredity, and the work that has 

 solved it is the main content of what is usually called genetics. 

 There is a second aspect of the problem of heredity: to under- 

 stand how the gene, whatever it is, acts in controlling typical 

 development to the adult form showing all the hereditary traits. 

 One might call this problem simply the problem of development. 

 But as development is to be linked specifically with the function 

 and action of the genes, this special chapter may be termed the 

 physiology of heredity; and the science devoted to its solu- 

 tion, physiological genetics. ( >ne might also speak of static genetics, 

 studying the status of the germ plasm; and of dynamic genetics, 

 inquiring into the acting forces that lead to the visible effect. 



The problems <>t' static genetics may be and have been attacked 

 directly, and the laws and rules that have been derived from 

 experiments in this field may actually be made visible in con- 

 comitant cytological studies. Physiological genetic-, however, 

 does not offer any method of direct attack and therefore calls 



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