CHAPTER XXV 



GENETIC CHANGES IN BISEXUAL REPRODUCTION 



How common are genetic changes in ordinary bisexual re- 

 production? This question also can be answered only in 

 relative terms. Few organisms have been studied intensively 

 enough and for enough generations in succession to enable 

 us to answer the question intelligently. Drosophila melano- 

 gasier has probably been studied more thoroughly than any 

 other species, these studies, because of the rapid reproduc- 

 tion of Drosophila, extending over hundreds of successive 

 generations. No other organism has yielded such a great 

 number of known distinct genetic variations, but at first 

 their discovery came rather slowly. Improved technique and 

 training on the part of observers enabled them to recognize 

 more and more genetic changes. Those discovered within 

 ten years have mounted in number into the hundreds. There 

 is reason to think that a goodly proportion of these genetic 

 changes have actually occurred (not merely been discovered) 

 during the period of laboratory study of Drosophila at Co- 

 lumbia University. Some of them have been observed to 

 occur independently at different periods and in unrelated 

 stocks of flies. This indicates that in the best-known genet- 

 ically of all organisms the genes are extremely numerous and 

 are subject to rather frequent changes, for we are acquainted 

 only with such genes as have revealed themselves by under- 

 going change. The first discovered gene was that for white 

 eye. In all eight different allelomorphic forms of this gene 

 have now been described, viz., (1) white, (2) tinged, (3) buff, 

 (4) eosin, (5) cherry, (6) blood, (7) coral, and (8) red. They 

 form a series of grades of increasing intensity of red pigmen- 

 tation, each one having made its appearance independently 

 of the others. Bridges has made an intensive study of minor 

 genetic variations in one of these seven grades, viz., eosin, 



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