MULTIPLE FACTORS 195 



very nearly normal in appearance. The degree of 

 abnormality and the proportion of abnormal off- 

 spring are both capable of being altered, within 

 limits, by selection or by crossing to normal stock. 

 Dexter crossed beaded flies to flies carrying mutant 

 factors in the different chromosomes and studied 

 the linkage of the beaded character with these other 

 characters. He found that beadedness showed link- 

 age 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 indicates 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 pre- 

 serving this factor. 



An extensive selection experiment was carried out 

 by Lutz on Drosophila. He selected for abnormal 

 wing venation chiefly for extra veins. Abnormali- 

 ties occur in nature in about 0.3 per cent, of the flies. 

 In two separate experiments Lutz increased this to 

 approximately 100 per cent, abnormals, and in one 

 of the experiments kept it there for eight generations. 

 But, in this same experiment, one pair (brother and 

 sister of the first pair that produced 100 per cent. 

 abnormals) produced no abnormals, and their de- 

 scendants remained for 40 generations a strain which 

 gave scarcely more abnormals than does a wild 

 strain. Possibly a mutation occurred here, although 



