A LINKAGE VARIATION IN DROSOPHILA 



CALVIN B. BRIDGES 



From the Zoological Laboratory, Columbia University 



In the breeding work upon Drosophila done in this laboratory, 

 it has been the practice to allow a female to lay eggs only for a 

 period of about ten days. This is the average length of time 

 from the mating of a female to the emergence of her offspring. 

 But this first brood does not by any means exhaust the eggs of 

 a female; if she is transferred to a fresh culture bottle she will 

 lay as many eggs in this as in the first, and will continue to lay 

 for forty or fifty days. Ordinarily, then, we obtain a sample 

 of from 200 to 400 flies from a female, although three times as 

 many might be obtained. It seemed to me that the full output 

 of each female would give a truer index than the one that we 

 were using. Accordingly, in working out the linkage relations 

 of several mutations, 1 raised from each of the Fi females of a 

 few experiments a second brood. 



In cases involving the second chromosome a remarkable 

 relation came to light when the results of the second broods were 

 compared with those from the first. There had been a change 

 in the linkage so that both in the totals for each experiment and 

 in a great majority of the individual cultures the percentage 

 of crossing-over had fallen significantly. Or, in other language, 

 the 'coupUng strength,' or 'gametic ratio' had risen. This 

 change, while very interesting theoretically, promises further 

 to become an aid in the study of the mechanism of linkage. 



In the case of the first (sex) chromosome a large amount of 

 data shows no change from first to second broods. 



In the case of the third chromosome present data show an 

 increase in the percentage of crossing-over, but because of the 

 small number of cases the rise may not be significant. 



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THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 19, NO. 1 

 JULY, 1915 



