SEX-LINKED LETHALS IN DROSOPHILA 103 



WI2M 

 wl_,2m 



b 



c 



W12M 



WLzm 



wLzM 

 Wlzm 



L 



w 2i ^ 



L 



w 



. WL2M 



' Figure C 



In this case there is crossing over between L2 and W and simul- 

 taneously between L2 and M. Expressed in another way this 

 means that the chromosomes cross between these two points, 

 and the parts on the same side reconnect at the place of cross- 

 ing. As a result of double crossing-over, the two gametes that 

 result are W L2 M and w I2 m, the latter of which never appears 

 in the males. 



If a female with these possibilities is crossed to a white miniature 

 male (i.e., a male that carries these two recessive characters) 

 the male classes will correspond to the gametic classes of that 

 female except for those that carry the lethal which are not realized. 

 The most frequent gametic class will be the non-cross-over class. 

 The frequency of the other classes will depend on the distance 

 of the factors involved from each other, because the chance of 

 the crossing of the chromosomes taking place will be in pro- 

 portion to the interval between the two loci. For this second 

 lethal factor, therefore, the surviving classes of males are the 

 following : 



Non-cross-over \v L2 m — white miniature (f 



) Single cross-over W L2 m — red miniature c?" 



\ Single cross-over w L2 M — white long cT 



Double cross-over W L2 M — red long cf 



