ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 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 allelomorphie 
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. 
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