360 GENETICS 



genes, there are less common dominant mutations, which 

 produce an effect even in the presence of the normal gene. 

 Such are in Drosophila the mutations abnormal abdomen, 

 bar-eye, notch wings and the like. These have an effect dif- 

 fering from that of the normal gene; they are not mere 

 inactivations. As a rule they produce a disturbance of devel- 

 opment, causing it to take an abnormal course. Almost all 

 such dominant genes are lethal when homozygous. It may 

 be remarked that such dominant mutations have of late been 

 shown to be commonly produced by breakage or other in- 

 jury to chromosomes. 



There are also a few cases of recessive mutations which 

 are not inactivations, but produce an effect differing from 

 that of the normal gene. An example of this is ebony body- 

 color in Drosophila. The mutated gene produces deeper pig- 

 mentation than the normal gene. 



Though in the overwhelming majority of cases mutations 

 are disadvantageous or can be proved to be partial inacti- 

 vations of the normal genes, in rare cases mutations have 

 been held to be advantageous. Thus, in the cotton plant 

 there are two varieties, one having forked or deeply cleft 

 leaves, the other leaves that are nearly entire, being but 

 slightly lobed. Horlacher and Killough ^^ observed that un- 

 der radiation a mutation from forked leaves to the lobed 

 leaves was produced. As the lobed leaves have more leaf 

 surface, the investigators hold that this mutation may have 

 been beneficial. 



Again in the production of mutations by heat, Jollos 

 found that the red eye of the fruit-fly sometimes mutates to 

 a white color, without pigment. This is a common mutation, 

 that occurs also under radiation and under natural condi- 

 tions. It is a typical loss mutation, resulting in a weakening 

 of the constitution, so that under ordinary conditions the 

 mutated flies show a higher mortality than the normal red- 

 eyed individuals. But Jollos finds that the white-eyed flies 



