ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 161 
allelomorphs as applied to the case in rabbits, for although the homo- 
zygous forms give such a series of diminishing intensity of melanic pig- 
ment, nevertheless the heterozygous forms give inconsistences. Black 
by agouti gives agouti-black, but black by yellow gives full black, in 
spite of the fact that yellow is regarded as a lower intensity of pigmenta- 
tion than agouti. 
The argument does not appear to be valid, however, for specific 
relations may still exist among the factors of a system of multiple allelo- 
morphs. Bridges has pointed this out in the case of the eye color series 
red, white, cherry, eosin, tinged, blood and buff eye-color in Drosophila. 
He has discovered a number of factors which modify eosin, one in partic- 
ular called whiting changes eosin to pure white, but does not produce 
any visible effect on the other members of the series. The conception 
of diminishing intensity as applied to multiple allelomorphs is clearly 
not fundamental to the hypothesis. 
2. The behavior in inheritance is different from that which would be 
expected in case different loci in the hereditary system were involved. 
When different loci are involved, each of two different mutant types will 
contain besides its own mutant factor the normal allelomorph of the 
mutant factor of the contrasted type. Consequently on crossing they 
will unite the series of factors present in the original type and give a 
character expression corresponding to that of the original form. Such 
is normally the case in undoubted instances of mutations affecting differ- 
ent loci, but in the case of multiple allelomorphs one or the other of the 
mutant types or an intermediate is produced in F;. When identical 
loci are concerned in two mutations, the hybrid between them will not 
_ reconstitute the original system, but will contain only the two mutant 
factors at that locus. The character expression of the hybrid therefore 
will depend on the interrelations existing between the mutant factors 
and the rest of the hereditary system rather than on the reuniting of the 
normal allelomorphs of the mutant factors. 
3. There are difficulties in explaining the origin of some of the forms 
on the basis of complete linkage between factors, which disappear on the 
adoption of the hypothesis of multiple allelomorphism. The difficulty 
may be illustrated by a specific case, that of the series red, white, cherry, 
eosin, tinged, blood and buff eye-color in Drosophila. Considering two 
specific instances, cherry and white, both of which arose from red immedi- 
ately, it must follow on the basis of complete linkage that one differs from 
red by one factor and the other by two factors. If red be (CE)(CE), then 
cherry, which is recessive to red would be (cH)(c#), and white, which is 
recessive to both red and cherry would be (ce)(ce). This involves the 
assumption that white arose as a result of simultaneous mutations in two 
completely linked factors affecting the same character, a practically 
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