Payne: Variations ojid Selection 



25 



means, of course, that neither of the third chromosomes 

 from the selection hne is present in a pink-eyed fly. As the 

 pink flies have few extra bristles, and as they get both third 

 chromosomes from the pink line, the probability is that there 

 is a factor for extra bristles in the third chromosome. 



An attempt was made to test this by means of linkage. 

 The selection line was crossed to the combination, sepia spine- 

 less kidney sooty rough. The genes for these five charac- 

 ters are located in the third chromosome. The F/s were 

 inbred. If crossing-over occurs, and if there is a gene for 

 extra bristle number in the third chromosome, it would be 

 possible to link it with one of these five. To my surprise, 

 however, practically no crossing-over occurred (see Table 

 VIII). I have not as yet tested out the possibilities for this 

 unexpected result, but presumably it is due to the presence 

 of some factor which modifies crossing-over. Factors of this 

 type have been listed by Morgan, Sturtevant, Muller, and 

 Bridges in their textbook as ''little-cross-over" and "low- 

 cross-over". So far as I know, a description of their behavior 

 has not appeared in print. This cross again brought out 

 the fact that all flies which get both third chromosomes from 

 a line other than the selection line, in this case, sepia spineless 

 kidney sooty rough, have fewer extra bristles. The analysis 

 of the cross between these two lines made a little differently 

 is suggestive. A female from the selection line was mated 

 to a sepia spineless kidney sooty rough male. The F^ males 

 were back-crossed to sepia spineless kidney sooty rough 

 females. The offspring of this mating are normal (wild type) 

 males and females and sepia spineless kidney sooty rough 

 males and females. Some of the normal males and females 

 have extra bristles. Also some of the sepia spineless kidney 

 sooty rough females have extra bristles, but none of the sepia 

 spineless kidney sooty rough males have extra bristles (see 

 Table IX). When the origin of the chromosomes of these 

 four groups of flies is studied, the following facts come to 

 light. The F^ male gets his X-chromosome and one member 

 of each of the second, third, and fourth pairs from the selec- 

 tion line. The other member of the second, third, and fourth 

 pairs comes from the sepia spineless kidney sooty rough line. 

 In the formation of the germ cells of this male there will be 

 two classes of spermatozoa so far as the X-chromosome is 

 concerned. One class will get it, the other will be without 



