No. 644] SHORTER ARTICLES AND DISCUSSION 287 



position is in the region of dicheate hairless of the third chromo- 

 some. It may be noted that such recessive factors effecting 

 the mechanism of segregation show what might be called de- 

 layed Mendelian results for the F, flies must be tested for their 

 linkage relations before anything can be said regarding the 



Complete linkage has been reported in but one other case. 

 Thus in 1912 Morgan showed that crossing over did not occur 

 in the second chromosome of the male of this same species, 

 meianogaster. This phenomenon has since been extended to 

 include the other chromosomes. If it be considered that cross- 

 ing over as originally discovered for the female of this species 

 is the normal, then Sturtevant has shown not less than three 

 dominant factors to materially reduce the normal amount of a 

 crossing over in the second and third chromosomes. A further 

 incompletely analyzed case of the same investigator suggests 

 that a third chromosome dominant partly controls an increase 

 in crossing over in the second chromosome. Crossing over 

 variations have been shown by Bridges in his "deficiency" 

 case, etc. From this it appears that there are three kinds of 

 effects shown by the crossover mechanism. The first case, that 

 of Morgan, shows no crossing over in the male. No genetic 

 factors have as yet been shown to be responsible for this. The 

 second case, that of Sturtevant, shows genetic dominant fac- 

 tors responsible for reducing crossing over in the female. The 

 third case, given here, shows recessive genetic causes allowing 

 no crossing over in the female. It further shows these factors 

 capable of acting on chromosomes of which they are not a part. 



Detlefesen and Roberts using the sex-linked factors, white 

 and miniature, present another kind of evidence. In a selec- 

 tion experiment they show crossing over to decline from the 

 normal amoimt (about 33 per cent.) to nearly zero per cent., 

 no evidence being presented as to the causative agent, although 

 the suggestion is made that "crossing over in the various 

 regions of the sex chromosome (and the other chromosomes!) 

 is probably controlled by multiple incompletely dominant fac- 

 tors." From what has been indicated above it seemed more 

 probable that recessive factors, perhaps one, are responsible 

 for these linkage variations. Especially is this true of their 

 results in series A and A\ for with delayed Mendelian segrega- 

 tion, recessive autosomal factors effecting crossing over in the 



