ALLELOMORPH1C RELATIONSHIPS IN MENDEL1SM 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 (cE}(cE], 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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