ALLELOMORPH1C RELATIONSHIPS IN MEN DELI SM 155 



the molecule or a change in an end radical, then it is not difficult to 

 imagine how a reverse mutation might arise. Reverse mutations, there- 

 fore, support the idea that the recessive member of an allelomorphic 

 system is just as truly a factor as the dominant member. Never- 

 theless these considerations do not in themselves confute the argument 

 of presence and absence, although they tend to throw the weight of 

 evidence strongly against it. It is, however, perhaps not amiss to point 

 out that much of the weight of authority of the presence and absence 

 hypothesis depends on the fact that it was advanced at the psycho- 

 logical moment, and that, as Morgan points out, in the light of our 

 present knowledge of the relation between factors and characters it 

 assumes a knowledge far beyond that which we have at present attained. 

 But the really serious objections to the hypothesis are those based on the 

 evidence furnished by multiple allelomorphism. 



Since the foregoing was written Bridges has published results of his 

 investigation of a case of loss or inactivation of a portion of the X-chro- 

 mosome in Drosophila. The deficient section involved the factor for 

 bar eye. As Bridges points out this constitutes the first valid evidence 

 upon the question of presence and absence. According to the presence 

 and absence hypothesis the original appearance of the dominant bar 

 character was due to the loss from the chromosome of an inhibitor, 

 thereby allowing the normal narrowing effect of the remaining complex 

 to assert itself. It should make no difference whether this inhibitor 

 were lost by a special loss involving only the inhibitor or whether it 

 were lost because of being situated in a particular section which became 

 lost. In other words, the chromosome which is deficient for the region 

 carrying the inhibitor should allow the occurrence of the same narrowing 

 effect that is allowed by the simple loss of the inhibitor. In point of 

 fact, the deficiency of the region in which the inhibitor must be hypoth- 

 ecated does not produce an effect like that of the mutation responsible 

 for bar. For, the female carrying one deficient X and one normal X 

 shows no narrowing of the eye shape, and likewise the female carrying 

 one deficient X and one bar X is no narrower in eye shape than a 

 normal heterozygous bar. Thus, in the only case which has a direct 

 bearing on the presence and absence hypothesis, it is seen that the ex- 

 pedient of the loss of inhibitors to explain the origin of a dominant 

 mutation is of no avail. 



Multiple Allelomorphism in General. Multiple allelomorphism is 

 the term applied to those cases which seem to depend on a series of 

 changes in a given factor locus. Cuenot advanced such an explanation 

 for the inheritance of certain color patterns in mice, and Morgan has 

 since described several cases which occur in Drosophila. Since these 

 later cases are simpler and have been worked out in more detail they will 

 be treated first. 



