64 GENETIC VARIATIONS 



It agrees well with this fact that most mutations are reces- 

 sive, as compared with the effect of the normal gene; so that 

 when the mutated gene and the normal gene are present 

 together, the effect on the characteristics of the organism is 

 that of the normal gene alone. There do occur, however, 

 mutations that are dominant; they produce a positive effect 

 different from that of the unchanged gene; an effect that is 

 manifested even when the unchanged gene is present. But 

 such dominant mutations not only as a rule are manifested 

 as marked imperfections or abnormalities, but also they are 

 seriously injurious to the constitution of the individual, so 

 that if two doses of the dominant mutation are present 

 usually the individual dies or fails to develop at all. 



On the whole it is certain that the overwhelming majority 

 of gene mutations are not adaptive, not progressive, but the 

 reverse. 



But are there no gene mutations that are beneficial to the 

 organisms in which they occur ? Are there none that leave the 

 organism more vigorous than before their occurrence? Are 

 there none that are progressive in character ? 



It is possible to collect from the literature of genetics a few 

 cases in which gene mutations are held to be possibly or 

 probably beneficial. 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 under radiation a mutation from 

 forked leaves to the lobed leaves was produced. As the lobed 

 leaves have more leaf surface, Horlacher and Killough hold 

 that this mutation may have been beneficial. 



Again, in the production of mutations by high tempera- 

 tures, one of the mutations is from the red eye color to white 

 eye; a common mutation also under radiation, and occurring 

 too under natural conditions. It is a typical loss mutation, 



