WHITE-OCELLI AN EXAMPLE OF A "SLIGHT' MU- 

 TANT CHARACTER WITH NORMAL VIABILITY. 



CALVIN B. BRIDGES. 



Most of the non-lethal mutant characters of Drosophila melano- 

 gaster may be described as "slight" in the amount of visible 

 change. This is fortunate, for, in general, the mutant genes 

 that cause great changes in an organ, or that cause several 

 distinct changes in different organs, produce races of poorer 

 viability than the wild type of the species. The amount of 

 disturbance to viability is roughly proportional to the extent of 

 the somatic change. In nature these "extreme" mutant types 

 would not be able to survive in competition with the wild type, 

 and in our experimental cultures their presence leads to aberrant 

 ratios that tend to obscure the simple genetic relations. It is 

 possible to reduce the shortage of these relatively inviable mutant 

 characters only by making the conditions of culture exceptionally 

 favorable. Thus, the number of eggs per culture must be re- 

 stricted by breeding from a single mother; and the amount of 

 food per culture should be carefully regulated. Great progress 

 has been made in improving the quality and methods of using 

 culture media. While it is possible by such improved methods 

 to make the relatively inviable mutants usable in many kinds 

 of experiments, there remain other experiments that require so 

 high a degree of exactness or the simultaneous use of so many 

 mutants that even one poorly viable mutant is inadmissible. 

 As the number of mutant types increases, we are able to drop the 

 use of more and more of the poorly viable mutants and to replace 

 them by new mutants of normal viability. As already implied, 

 these mutants of normal viability are nearly all characterized 

 by somatic changes of slight degree, or by effects, so far as observ- 

 able, upon a single organ only. Many of these "slight" char- 

 acters are perfectly definite in demarkation and offer as great 

 precision of classification as do more extreme mutant characters. 

 Another great advantage of slight mutant characters is that they 



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