12 



GENERAL RELATIONS IN THE OPERATION OF 



THE GENETIC SYSTEM THROUGH THE 



PASSAGE OF GENERATIONS 



The preceding chapters treat in detail the structure and 

 operation of the genetic system in reproduction and inherit- 

 ance. The present chapter deals with general relations which 

 are obscured in dealing with details, relations to be observed 

 when the units of observation are the successive generations. 

 To present these will require recapitulation in brief of some 

 matters already treated. 



Biparental reproduction is a great process of producing 

 new combinations of genes, and consequently new combina- 

 tions of characteristics. Through it the gene combinations 

 present in any generation are taken apart and combined 

 anew, giving individuals having diverse characteristics from 

 those present in the earlier generation. This is the process 

 known as Mendelian heredity. One result of the process is 

 that individuals of the later generation show on the average 

 a greater similarity in characteristics to their own immediate 

 parents than to other members of the earlier generation; 

 also they show a greater resemblance to other descendants 

 of those parents than to the rest of the population. These 

 resemblances have been seized upon as of special interest, 

 and in view of them the terms heredity and inheritance have 

 been applied to the general process. But it is a process that 

 produces diversities as well as similarities; it produces di- 

 versities on a grand scale. 



Each individual, as we have seen, carries a definite com- 



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