336 GENETICS 



relation to other genes, presumably on the Interaction of 

 genes that are close together. This is spoken of as a "posi- 

 tion effect." '^ According to this suggestion, when a gene 

 changes its position with relation to other genes, it may 

 change its method of action. In one position its effect is to 

 produce what is commonly called the normal or unchanged 

 characteristic. In another position, it becomes weak and 

 unstable in its action, or it produces a different effect on the 

 characteristic, an effect that has been called a mutation. 

 This matter will be discussed further in the chapter on mu- 

 tations. 



Changes in the arrangement of the genes within the ge- 

 netic system appear to occur somewhat frequently in certain 

 plants. In Datura and Oenothera it is found that certain 

 stocks differ from others in the fact that chromosomes of 

 different pairs have exchanged parts, as happens in the 

 translocations induced by radiations. In this way have been 

 produced a considerable number of different stocks, having 

 the same genes, but with the genes differently combined in 

 chromosomes in the different stocks.^ Usually such differ- 

 ences produce, in these plants, no difference in the action of 

 the genes, a fact that appears inconsistent with any general 

 validity of the hypothesis of a "position effect." The diverse 

 races differ of course to a certain extent in the linkage of 

 the genes. 



It thus turns out that in some species, at least, the way 

 the genes are arranged in the chromosomes is less stable 

 than had been supposed. Blocks of genes have been shifted 

 about from one chromosome to another, without causing 

 any marked change in the action of the genes. 



In another way changes in the grouping of the genes ap- 

 pear to have played a part in producing diverse varieties 

 or species. In any given variety or species, the genes are 

 grouped into a certain definite number of chromosomes: in 



