SHORTER ARTICLES AND DISCUSSION 



AN INTKINSIC DIFFICULTY FOR THE VARIABLE 

 FORCE HYPOTHESIS OF CROSSING OVER 



The assumption of a " variable specific force, ' ' made by Gold- 

 schmidt/ may seem to account for the frequency of the crossovers 

 occurring in a given simple case of linkage; but when this ex- 

 planation is extended to the results which such crossovers give 

 when bred, it creates a difficulty of the same type and magnitude 

 as the original problem of crossing over, for which, therefore, it 

 is not a satisfactory solution. 



Briefly put, the explanation advanced by Goldschmidt as- 

 sumes that the genes are carried by the chromosomes, and that 

 each gene is incorporated in its characteristic locus by virtue of 

 a force residing in the chromosome and possessing properties 

 specifically related to the properties of the genes of that locus. 

 In the heterozygote Gg (see accompanying figure, line 1), the 

 two forces Fg and Fg residing in the homologous chromosomes C 

 and C possess not only a locus specificity but also an allelomorphic 

 specificity corresponding to the allelomorphs G and g. When the 

 chromosomes of the Gg heterozygote go into a resting stage, these 

 forces P<3 and Fg relax, so that the genes G and g become freed. 

 When the chromosomes are reassembled preparatory to division 

 these forces again come into play with the result that gene G 

 is again incorporated into the chromosome in which Fq resides, 

 while gene g is likewise reincorporated into the homologous 

 chromosome characterized by the presence of Fg. In order that 

 crossing over may occur, the allelomorphic specificities of forces 

 Fq and Fg must, in the first place, be commensurable variables; 

 i. e., forces F^ and Fg must vary in that property which con- 

 stitutes their essential difference, and in such a manner that 

 when all the values of force F^ are represented by a character- 

 istic frequency distribution and likewise all the values of Fg are 

 represented by a second specific distribution, these two distri- 

 butions will have a common base (see diagram, line 1). In the 



base line so that a value chosen from the lower range of one may 

 iDr. E. Goldschmidt, "Crossing over ohne Chiasmatypie? " Genetics, 2: 



