LINKAGE IN PEROMYSCUS 



DE. F. B. SUMNER 

 The Scripps Institution for Biological Research, La Jolla, Calif. 



Students of Mendelism are beginning to display the 

 same interest in possible homologies between the genetic 

 factors or "genes" of different species of animals or 

 plants which the morphologists of thirty years or more 

 ago did in homologies between organs. In considering 

 a given case of suspected homology between genes, two 

 criteria are, so far as I know, employed: (1) Resem- 

 blance between the developed characters which are at- 

 tributed to the action of supposedly homologous genes. 

 Mere similarity of appearance, however, is recognized 

 as an extremely fallible criterion of homology here as 

 in the case of comparative anatomy. (2) Agreement be- 

 tween the ''cross-over" value shown by a pair of linked 

 factors in one species, as compared with the correspond- 

 ing value shown by supposedly homologous factors in 

 another species. If both of the two linked genes under 

 consideration are found to have much the same somatic 

 effects in the two species, and if, furthermore, the de- 

 gree of linkage is approximately the same in the two 

 cases, the argument is strong for a twofold homology. 



Metz^ and Sturtevant^ have been investigating the 

 parallel mutations of several species of Drosophila, and 

 it is not unlikely that this genus will furnish the best 

 material for the study of genie homologies, just as it 

 has shown incomparable superiority for certain other 

 lines of genetic research. 



For rodents, what appear to be parallel mutations have 

 been shown to occur among numerous species, even ones 



1 Genetics, March, 1918. 



2 Genetics, January, 1921. 



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