248 MULTIPLE FACTORS 



mutant factors in the different chromosomes and 

 studied the hnkage of the beaded character with 

 these other characters. He found that beadedness 

 showed Hnkage to third-chromosome characters, 

 indicating that there is at least one factor for the 

 character located in that chromosome. He also 

 found that sometimes beadedness showed linkage 

 to second-chromosome characters, while at other 

 times it failed to do so. This indicated that the 

 beaded stock was impure for a factor located in the 

 second chromosome, which when present increases 

 the amount of beading. Selection would be effective 

 either by eliminating or by preserving this factor. 



Later, Muller found, by means of linkage experi- 

 ments, that the chief factor for beaded, lying in the 

 third chromosome, is lethal when homozygous, but 

 that the highly selected heterozygous stock also 

 carries another lethal, lying in the homologous 

 third chromosome in almost complete hnkage with 

 the normal allelomorph of beaded. Neither beaded 

 nor its normal allelomorph, therefore, can exist in 

 homozygous condition, and the stock breeds true 

 to its heterozygous type. Since this ''balanced 

 lethal" stock had at the same time become homo- 

 zygous for the intensifier in the second chromosome, 

 it resulted that, although heterozygous for the 

 chief factor, it had become ''pure" for the character 

 beaded. Further results with this stock will be 

 considered in the last chapter. 



An extensive selection experiment was carried 

 out by Sturtevant on the character "dichaete," 



