202 GENETICS IN RELATION TO AGRICULTURE 



will make this matter clearer. Bridges took non-disjunctional females 

 known from the type of mating involved in their production to be of 

 the genetic constitution (WVFb / X)(w e vfb'X)Y and mated them to bar- 

 eyed males (WVFB'X)Y. Obviously the regular daughters of such a 

 mating will be bar-eyed, because they receive from the father an X- 

 chromosome bearing the dominant factor for bar eyes, but the excep- 

 tional daughters will not be bar-eyed since both their X-chromosomes 

 are derived from the mother. The question concerning these excep- 

 tional daughters is as to whether they are invariably of the genetic 

 constitution (WVFb'X)(w e vfb'X)Y or whether they may occasionally 

 be cross-overs, for example (WVfb'X)(w e vFb'X)Y or (Wvfb'X)(wVFb f - 

 X)Y. Since the loci involved in this case are W = 1.1, V = 33.0, 

 and F = 56.5, normal crossing-over should give about 50 per cent, of 

 cross-overs. By testing the exceptional females again with bar males 

 of the above genetic constitution, the distribution of the males into 

 phenotypes serves as an accurate indication of the genetic constitution 

 of the mother. In every case in tests of thirty-seven exceptional daugh- 

 ters, wild type males (WVFb'X}Y and eosin vermilion forked males 

 (wvfb'X) made up the largest classes. This indicated that the females 

 were all of the genetic constitution (W VFb'X)(w e vfb'X)Y, and, there- 

 fore, were non-cross-overs. 



The above facts are to be taken in conjunction with the fact that 

 crossing-over actually may occur in non-disjunctional females in homo- 

 synapsis. We have pointed out in another place that crossing-over 

 does not occur in males. Now in non-disjunctional females the occur- 

 rence of heterosynapsis might well set up a condition like that which is 

 responsible for non-crossing-over in the male for we would have duplicated 

 the exact type of reductional divisions which occur in the male aside 

 from the presence of an unpaired Jf -chromosome in the reduction spindle. 

 But as a matter of fact the presence of the F-chromosome does not appear 

 to affect crossing-over between the .X-chromosomes in homosynapsis. 

 Thus Bridges has summarized the data for crossing-over in non-dis- 

 junctional XX Y cultures and compared them with the data for crossing- 

 over in normal XX cultures with the results given in Table XXXVI. 

 Far from resulting in no crossing-over the presence of the F-chromosome 

 actually appears to have increased the per cent, of crossing-over be- 

 tween loci in the X-chromosomes. No reason can be readily assigned 

 for this increase in crossing-over, but it is of interest to note that the 

 presence of a F-chromosome does not preclude the occurrence of cross- 

 ing-over. 



In Fig. 91 it is shown that half of the regular sons of a non-dis- 

 junctional female are of the type XYY instead of XY as normally. The 

 hereditary behavior of such males as determined by experiment is shown 



