CHAPTER II 



(i) Genetic Variations Resulting from Irregularities in 

 THE Structure and Operation of the Genetic System 



(2) Gene Mutations 



IN SEARCHING for the method by which new kinds of 

 genes are produced, investigators have come upon another 

 type of genetic variation. The genetic system is a mechanism, 

 operating in a complicated fashion. As the cells divide and 

 other operations occur, the genetic system appears under the 

 microscope to be of an astonishingly machine-like character. 

 And like other mechanisms it is subject to getting out of 

 order, to accidents, to breaks and irregularities of operation. 

 When it fails thus to act in the expected manner, it causes 

 changes in the characteristics of the individuals that are 

 produced; it yields genetic variations.^ 



This production of genetic variations by breaks and irregu- 

 larities in the genetic system plays at present a great role in 

 genetic science, and it seems destined to play a still greater 

 role in the future; one till recently unsuspected. 



As you recall, the different materials of the genetic system — 

 the genes — are strung up in long strings (Figures i and 2). 

 These strings are divided into segments, which in their con- 

 densed condition we call the chromosomes. Since a set of 

 genes comes to the individual from each of his two parents, 

 there are two sets of genes, and therefore two sets of chromo- 

 somes in his cells. The chromosomes, like the genes of which 

 they are composed, are therefore in pairs; one member of 

 each pair derived from the individual's father, one from his 

 mother. 



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