232 DANIEL ROSEN 



an important rôle in the interchange of the specific genes in the 

 chromosomes, and that affinity controls crossing over at the places 

 of contact during synapsis — tlie stage of crossing over according 

 to Morgan. 



It has already been held forth that the genes are placed behind 

 one another in a row. Granted that the genes, or the chromosome 

 parts of the genes, possess a marked, mutual affinity the assumption 

 lies close at hand that they also are mutually arranged according to 

 their degree of affinity. A certain distance between them is then to 

 be expected. Assuming a difference in affinity between different 

 genes the question, however, may be raised if not the different 

 »distance» between contiguous genes may be replaced by the assump- 

 tion of different degrees of affinity but identical distances between 

 the genes. Instead of assuming the distance between two genes to 

 be two times the distance of a second pair the assumption may be 

 made that the distances are identical, while the difference of affinity 

 between the two first mentioned genes is two times the difference of 

 affinity between the two latter. It is then to be expected that the 

 two first mentioned should be more loosely connected allowing a force 

 of the magnitude X to break them apart, w'hile a force of the 

 magnitude 2 X is required to break the latter ones. We will assume 

 that the genes A and ß. C and D are placed in a row behind one 

 another and that the distances between them are identical, while the 

 differences in affinity between them are such that for breaking of 

 the combination between C and D the force X is required, for breaking 

 of the combination between ß and C the force 2 X is required and 

 for breaking of A and B the force 3 Z is needed. Granted that these 

 forces are at work upon one or the other combination at different 

 moments the following results are obtained: 



1. In case of a force X working upon the combination CD breaking 



We thus obtain breaking of the combination CD in 3 cases, of 

 the combination BC in two cases, and of the combination AB in one 



