TWO SEX-LINKED LETHAL FACTORS IN DROSOPHILA 

 AND THEIR INFLUENCE ON THE SEX-RATIO 



T. H. MORGAN 



From the Zoological Laboratory, Columbia University 



SEVEN FIGURES 



By a recessive lethal factor I mean any factor that brings about 

 the death of the individual in which it occurs, provided its effect 

 is not counteracted by the action of its normal allelomorph. 

 The term is not intended to mean that some poison is produced 

 that destroys the individual, but rather some defect is inherited 

 that is serious enough to render the individual unable to live. 

 The defect may be a physiological defect or a morphological 

 malformation; in fact, a defect in any organ essential for the life 

 of the larva, pupa, or imago would come under the general cate- 

 gory of lethal. In the present case I have not attempted to 

 discover where the elimination of the individual occurs, or to 

 what specific defect it is due. 



Lethal factors may be sex-linked or not, i.e., they may be 

 carried by the sex chromosome or by an autosome. If the lethal 

 factor is sex-linked it will kill any male in which it occurs, since 

 the male has but one X chromosome. Such a factor can, there- 

 fore, never be transmitted through the male Une, and, as a conse- 

 quence, it is not possible to transfer a lethal factor from one 

 lethal stock to another lethal stock, and in this way get two 

 lethals together, because, as we have seen, all males that contain 

 a sex-Unked lethal factor die. If two sex-linked lethals should 

 ever occur in the same female one must have arisen as a muta- 

 tion independently of the other. 



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