248 MULTIPLE FACTORS 
mutant factors in the different chromosomes and 
studied the linkage of the beaded character with 
these other characters. He found that beadedness 
showed linkage 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 linkage 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,”’ 
