THE CHROMOSOMES Jl 



haps failures of linkage might be due to the fact that in 

 synapsis there was an exchange of material between the 

 homologous chromosomes, that synapsis was in fact a 

 sort of "shuffle and cut" process. Should this be true, 

 it might be expected that if the factors or determiners 

 occupied definite places in the chromosome, those near- 

 est together would be least likely to be separated. This 

 hypothesis was tested by the most elaborate breeding 

 experiments, and eventually the relative positions in the 

 chromosomes of many factors were determined. The 

 results not only agreed with the hypothesis, but served 

 to confirm it. Thus if the relative positions of A and 

 B were calculated, and then those of B and C, it followed 

 that A and C ought to behave in a certain way when 

 brought together in a cross, and predictions of this sort 

 were fulfilled in numerous instances. It was found that 

 some factors crossed over less than once in a hundred 

 times ; others as often as once in every other time. In 

 the latter cases the factors lie far apart, probably near 

 the opposite ends of the chromosome. Very recently LOSS of part 

 Mr. C. B. Bridges has been able to show that in a par- 

 ticular case, instead of an exchange of substance, a 

 piece out of a chromosome was lost. In an experiment 

 with Drosophila flies, a particular character which 

 should have appeared, according to the known char- 

 acters of the ancestors, failed to develop. It occurred 

 to Bridges that if it, or rather the determiner for it, had 

 really got lost, very possibly other determiners, known 

 to lie very close to it in the chromosome, had also gone. 

 He tested this by further breeding, and found it to be 

 the case. Thus he at once confirmed his idea concern- 

 ing the loss of a fragment, and furnished additional 

 proof of the theory concerning the position of the de- 

 terminers in the chromosome, 



