MUTATIONS 267 



and the phenomenon of" substitutional changes of atoms or radicals 

 by which such complex compounds are transformed, we can express a 

 conception of the nature of factor mutations. 



To be specific let us suppose that some unusual condition occurs in a 

 certain germ cell of a normal female Drosophila such that a single atom 

 in each of the very complex molecules of the substance unique for the 

 locus W in the X-chromosome changes place with a different atom in 

 the surrounding nucleoplasm the substance unique for the locus W 

 is no longer capable of conditioning the laying down of red pigment in 

 the eyes and, if the affected ovum is fertilized by a F-bearing sperm, 

 a white-eyed male appears, the result, as we say, of a factor mutation. 

 This conception of factor mutations is useful as a basis for the multiple 

 allelomorph hypothesis. In order to explain how two or more factors 

 may have the same locus in a chromosome, it is only necessary to assume 

 as possible the substitution of two or more different atoms or radicals 

 in the molecule of the complex organic substance unique for the given 

 locus by other atoms or radicals in the nucleoplasm. 



Factors are relatively stable entities however. It has been shown al- 

 ready that any organism must possess thousands of factors, yet mutations 

 are comparatively rare even in Drosophila. These facts are rather 

 difficult to harmonize with our conception of the nature of factor muta- 

 tions. If substitutions of atoms or radicals occur why do they not take 

 place more frequently? Such questions must remain obscure until we 

 know something about the chemical constitution of the hereditary factors. 

 Only then can we expect to understand clearly the nature of the altera- 

 tions which occasionally are made in them. 



In this connection the behavior of factor mutations in inheritance is of 

 decided interest. As a rule they are recessive to their normal allelo- 

 morphs and for some time they were thought to be due to the loss of 

 factors, this idea being associated with the presence and absence hypothe- 

 sis. But on rare occasions dominant mutations have appeared. Among 

 150 mutations from the normal type of Drosophila ampelophila several, 

 such as bar eye, dark streak on thorax, abnormal abdomen and CIII, a 

 factor which modifies eosin eye color, are dominant over their respective 

 allelomorphs. A few other mutant characters have been found to be 

 dominant, such as hornlessness in cattle and red buds in the evening prim- 

 rose ((Enothera rubricalyx), but the great majority are recessive as is 

 indicated by the ratio in F 2 from crosses between mutants and normal 

 individuals. The condition in F\ by no means always indicates complete 

 dominance of the normal character. Hence it is clear that whatever the 

 nature of the mutation-producing chemical change may be, as a rule it is 

 either completely subordinate to the normal condition or else it merely 

 modifies the effect of the normal state in heterozygous individuals, making 



