13 



GENETIC VARIATIONS 



Changes in the Structure and Operation of the Genetic 



System 



The intensive studies of the genetic system and its opera- 

 tion that have been made in the last thirty-five years have 

 shown that this system may become changed as time passes 

 and that these changes give rise to alterations in the in- 

 herited characteristics of organisms. Such changes may be 

 called genetic variations. Genetic variations of a number 

 of different types are known, and there may be other types 

 as yet unknown. The known types represent changes in the 

 structure and operation of the genetic system — of the 

 chromosomes and genes. 



The genetic system presents three diverse aspects, each 

 of which may become altered, so that each of the three 

 yields a distinctive type of genetic variation. The genetic 

 system, as we have seen, is composed of many diverse or- 

 ganic materials, the genes, arranged in definite structures, 

 the chromosomes, which together form a complex appara- 

 tus. In different individuals this apparatus has different com- 

 binations of genes. 



I. Changing the combination of genes that are present 

 in the system gives one type of genetic variation. This oc- 

 curs in the normal processes of biparental reproduction and 

 Mendelian inheritance. We have already given an account 

 of this, and of its consequences, in Chapter XII. We there 

 saw that it may give rise to new combinations of character- 



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