GENE MUTATIONS 43 



inheritance. It must be shown farther that the change in the 

 genetic system that gives rise to the change in characteristics 

 is limited (at least mainly) to a single one of the genes; and 

 that it is not simply the loss of a gene. These conditions are 

 not by any means easy to fulfil. They can be fulfilled only in 

 organisms in which there has been acquired a detailed knowl- 

 edge of the structure and operation of the genetic system, so 

 precise that the locations of the single genes can be deter- 

 mined; knowledge that would permit maps to be made, 

 showing the relative positions of the genes. The effects of 

 particular genes on the characteristics must be known, so that 

 the effect of changes in these genes can be detected. Above 

 all, the results of typical recombinations among the genes, 

 such as occur in biparental reproduction, must be known. 

 And finally, the results of chromosomal irregularities must 

 have been adequately investigated. Until these things are 

 known, it is impossible to distinguish the results of recombi- 

 nations and irregularities, on the one hand, from the actual 

 changes in single genes on the other. There are very few 

 organisms in which these conditions have been even approxi- 

 mately fulfilled. The best known of these is Drosophila 

 melanogaster, the fruit fly. As will appear in the sequel, it 

 seems doubtful whether there has been hitherto any organism 

 whatever of which knowledge was adequate for distinguish- 

 ing gene mutations from other kinds of changes. 



However, many changes have been observed that affected 

 mainly individual genes, and that are commonly called gene 

 mutations. Whatever their character and whatever their rela- 

 tion to evolution, they are in themselves of great interest and 

 importance. A great body of detailed knowledge on these has 

 accumulated. We shall first deal with the main features of 

 this established knowledge. Next we shall make an explicit 

 examination of the question that is of underlying interest for 



