BRISTLE INHERITANCE IN DROSOPHILA 



I. EXTRA BRISTLES 



EDWIN CARLETON MACDOWELL 



Station for Ex-perimental Evolution of the Carnegie Institution of 

 Washington 



From the Osborn Zoological Laboratory of Yale University, and the Marine Bio- 

 logical Laboratory at Woods Hole, Mass. 



SIX FIGURES 



The four bristles normally found on the dorsal surface of the 

 thorax of Drosophila ampelophila form a rectangle. The extra 

 bristles studied in the following experiments occur in the two 

 longitudinal rows of the normal bristles, or just mediad or 

 laterad to these rows. Figure 1, showing some random patterns 

 of bristles, gives some idea of the different arrangements possible 

 for the most common numbers of extra bristles. These patterns 

 were drawn free hand, since the overlapping of the bristles 

 prevented the use of a camera. The drawings are not exact, 

 but the relative positions in the two rows are sufficiently accurate 

 to indicate that it would be practical" y impossible to make more 

 than a roughly approximate classification of flies according to 

 patterns, even if a very large number of standard patterns were 

 established. There appears to be no position in the two rows 

 that may not be occupied by an extra bristle, thus making it 

 impossible to develop any satisfactory homologies. This is 

 especially true in the higher grades. For these reasons the best 

 method seems to be recording merely the numbers of extra 

 bristles. As will be seen, the results based on the data of bristle 

 numbers shows very plainly that any attempt to obtain and use 

 data of patterns of extra bristles would be futile with the present 

 cultural methods. 



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