3l6 GENETICS 



istics, to characteristics increased or decreased In dimen- 

 sions or intensity beyond what before existed, and to new 

 characteristics. It is the commonest and one of the most im- 

 portant kinds of genetic variation. 



2. A second method of producing genetic variation is 

 by changes in the structure of the genetic system.^ The 

 numbers of chromosomes present may be altered. The 

 chromosomes may become broken into pieces, or united to- 

 gether in various ways. Genes may be lost. The arrange- 

 ment of the genes in the chromosomes may be changed. 

 Genes may be transferred from one chromosome to an- 

 other. All such changes cause alterations in the inherited 

 characteristics, or in the method by which the character- 

 istics are inherited. 



3. A third type of genetic variation arises from changes 

 in the physiological action of single genes, resulting either 

 from a chemical change in the constitution of the gene, or 

 from other causes. Such changes in the action of single 

 genes are commonly called gene mutations. 



Genetic variations resulting from gene recombinations 

 have been dealt with in Chapter XII. Here we take up the 

 other two types of genetic variation. 



Genetic V ariations Resulting from Changes in the 

 Structure and Operation of the Genetic System 



The genetic system, as we have seen, is a mechanism 

 operating In a complicated fashion. Like other mechanisms, 

 it is subject to getting out of order: to accidents, breaks, 

 and irregularities. Such accidents and Irregularities may 

 make impossible the operation of the genetic system, and 

 so cause death, or sterility. But in many cases the system 

 continues operating, but in a changed manner, and this 

 gives rise to changes In the characteristics that depend on 

 the system. Changes so produced are now under Investiga- 



