N on- Disjunction in Drosophila. 



39 



half of the primary exceptions survive. The XXY females are 

 fertile and can be used for further breeding. 



Non-disjunction also occurs in the male, giving XY and O 

 sperm. The XY sperm would produce XXY daughters, like the 

 XXY females above but with red eyes. They are therefore not 

 exceptions, but will give exceptions in their offspring. Wilson (1909) 

 observed three spermatocytes of Metapodius, another insect, in 

 which X and Y went to the same pole in the reduction division. 

 The XXY females give about 4% of secondary exceptions (i.e., 4% 

 of daughters like the mother and 4% of sons like the father) when 

 crossed with a male having other sex-linked characters. This was 

 Eggs XX XY X Y 



Sperms, X Y 



XXX 



(5) 9 vermilion (6) 3 vermilion (7) <J vermilion (8) dies 



exception 



Fig. 2. Offspring of XXY females. 



at first supposed to be due to a factor for non-disjunction. The 

 exceptions were afterwards found to be due to the presence of the 

 Y. After further breeding experiments it was predicted that 

 eytological examination of the daughters of an exceptional female 

 would show the presence of an extra chromosome in half the 

 daughters while the other half would be normal. This prediction 

 was completely verified. 



Let us consider now what will happen in XXY females. 

 Synapsis may take place between the two X chromosomes, leaving 

 the Y unmated (homosynapsis), or it may be of the XY type 

 (heterosynapsis). About 84% of the cases were found by breeding 



