CHAPTER 19 

 GENETICS 



In the chapters on Development the student has become ac- 

 quainted with the known facts concerning the processes of cell 

 division and differentiation in the origin of a new individual. We 

 may now consider the origin of new individuals with respect to 

 their resemblance to their parents. It is often said that like 

 reproduces like; and inheritance, in the biological sense, means 

 that offspring are similar to their parents. Members of a new 

 generation, however, always differ from the generation from which 

 they arise ; they also differ among themselves, except in the case of 

 " identical twins." Even in such instances a careful observer will 

 be able to detect a certain lack of exact resemblance. Such dif- 

 ferences between individuals that are in general alike are known as 

 " variations." We see, therefore, that in the reproduction of new 

 generations the offspring resemble and yet are unlike the parents. 

 The phenomena of inheritance and variation go hand in hand. 

 Heredity is, in fact, sometimes defined as " correspondence of 

 deviations from type," which might be expressed as a correspond- 

 ence in variations from the general likeness between generations. 

 Genetics is that part of biological science which deals with the facts 

 and theories of heredity and variation in the origin of new individ- 

 uals from generation to generation. 



It has been pointed out in previous pages that new individuals 

 arise from preexisting individuals by the process of asexual or 

 sexual reproduction. Sexual reproduction followed by an orderly 

 course of development, consisting essentially of growth and 

 differentiation, gives rise to a new individual which is hke, as well 

 as unlike, its parents. Whatever it is that determines the course of 

 development must determine the heredity and variation of the 

 individual. In other words, the offspring inherits a course of 

 development which under normal environmental conditions will 

 make it Hke the parents. A discussion of the theoretical concep- 



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