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 Y-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 1s 
indicated by the ratio in /’, from crosses between mutants and normal 
individuals. The condition in Ff, 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 
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