X AND Y IN DROSOPHILA 371 



three ways. First, Kaufmann (1933) has found that the attached 



XX become detached in XXY females by crossing-over between 

 X and y, such that two new chromosomes are produced, an X 



attached to a short arm of the Y, called XY^, and an X attached 



to the long arm of the Y, called XY^ (cf. D., 1935 e). Secondly, 

 Philip (1935) has shown that the reciprocal crossing-over of normal 



Fig. 116. — Behaviour of autosome and X-Y bivalents at meiosis 

 in cJ Drosophila pseudo-obscura. The autosomes are paired 

 at the centromere in diakinesis, the sex chromosomes by- 

 reciprocal chiasmata either in the short arm or the long arm 

 of Y. (D., 1934, cf- Fig- 63) 



males may sometimes take place on opposite sides of " bobbed," 

 which adjoins the inert region, so that this gene may be transferred 

 from X to y and vice versa without any other observable change. 



Finally, the association of X and Y chromosomes has been seen 

 in a pairing segment which lies on either side of the centromere in 

 D. pseudo-obscura (D., 1934 ; Dobzhansky, 1934). That the 

 association was in fact by reciprocal chiasmata could be shown 

 as follows : The unchanged X and Y separate at anaphase, having 



