SBX-LINKED LETHALS IN DROSOPHILA 



83 



A comparison of tables 2 and 3 brings out the very striking 

 fact that all the lethal sex ratios are characterized by the almost 

 complete disappearance of a particular class of males — the red- 

 eyed males, and that the absence or the smallness of this class 

 is a surer index of lethal cultures than is the sex-ratio itself! 

 Just as the red male class is almost exterminated, the white male 

 class is almost undiminished, i.e., it is of about the size of each 

 female class. That it is not nearer in size to the female class is 

 due to the relatively poorer viability of these males. 



This disproportionate effect of the lethal upon the size of the 

 two male classes is due to association, i.e., the loci occupied 

 resjiectively by the lethal and the white factors are very close 

 together in the sex-chromosome. It would require a break of the 

 chromosome between these two points to produce a red male 

 which lives (non-lethal) or a white male which dies (lethal), and 



TABLE 1 



The offspring of twelve red-eyed daughters of M, and of six red-eyed daughters of 

 K when mated separately to white-eyed males 



