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The chemical basis of heredity 



Heredity is that process which brings about the biological similarity 

 between parents and progeny. Genetics is the science of heredity. Its 

 subject matter is the multitude of events which determines the reappear- 

 ance of parental traits among progeny. 



These events can be grouped into three categories dealing, first, with 

 the nature of hereditary materials; second, with their replication and 

 transmission from generation to generation; and third, with the transla- 

 tion systems by means of which the hereditary materials exert control 

 over metabolism and development. These three categories are closely 

 interrelated, but to some extent they can be considered independently. 

 In this book Chapters 1, 2, 8, and 9 deal primarily with the identification 

 of hereditary determinants; Chapters 3-7 with replication and trans- 

 mission; and Chapters 10 and 11 with translation; in Chapters 12 and 13 

 some problems are discussed in which these categories are merged. 



Historically, the methodology of genetics has been based not on the 

 analysis of parent-progeny similarities but rather on differences, and to 

 a considerable extent this is still the rule. The method consists essen- 

 tially of mating unlike parents and analyzing the patterns of segregation 

 of parental differences among the offspring. Some parental traits are not 

 inherited at all, some give very complex patterns of segregation, but some 

 segregate in a remarkably clear-cut manner. It was the recognition of 

 this latter class which led to the explosively rapid development of 

 genetics. 



When two parents differ by a number of clear-cut traits, all progeny 

 will be readily classifiable and will show regular patterns of segregation 

 of each of these traits. When segregations are regular, predictions can 

 be made about the outcome of future matings, and the method can be 



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