14 SEX-LINKED INHERITANCE IN DROSOPHILA. 



SEX-LINKED LETHALS AND THE SEX RATIO. 



Most of the mutant types of Drosophila show characteristics that 

 may be regarded as superficial in so far as they do not prevent the 

 animal from living in the protected life that our cultures afford. Were 

 they thrown into open competition with wild forms, or, better said, 

 were they left to shift for themselves under natural conditions, many 

 or most of the types would no doubt soon die out. So far as we can see, 

 there is no reason to suppose that the mutations which can be described 

 as superficial are disproportionally more likely to occur than others. 

 Of course, superficial mutations are more likely to survive and hence 

 to be seen; while if mutations took place in important organs some of 

 them would be expected to affect injuriously parts essential to the life 

 of the individual and in consequence such an individual perishes. 

 The "lethal factors" of Drosophila may be supposed to be mutations 

 of some such nature; but as yet we have not studied this side of the 

 question sufficiently, and this supposed method of action of the lethals 

 is purely speculative. Whatever the nature of the lethals' action, it 

 can be shown that from among the offspring obtained from certain 

 stocks expected classes are missing, and the absence of these classes 

 can be accounted for on the assumption that there are present mutant 

 factors that follow the Mendelian rule of segregation and which show 

 normal linkage to other factors, but whose only recognizable difference 

 from the normal is the death of those individuals which receive them. 

 The numerical results can be handled in precisely the same way as 

 are other linkage results. 



There are some general relations that concern the lethals that may 

 be mentioned here, while the details are left for the special part or are 

 found in the special papers dealing with these lethals. A factor of this 

 kind carried by the X chromosome would be transmitted in the female 

 line because the female, having two X chromosomes, would have one of 

 them with the normal allelomorph (dominant) of the lethal factor 

 carried by the other X chromosome. Half of her sons would get one 

 of her X's, the other half the other. Those sons that get the lethal 

 X will die, since the male having only one X lacks the power of con- 

 taining both the lethal and its normal allelomorph. The/other half 

 of the sons will survive, but will not transmit the lethal factor. In 

 all lethal stocks there are only half as many sons as daughters. The 

 heterozygous lethal-bearing female, fertilized by a normal male, will 

 give rise to two kinds of daughters; one normal in both X's, the other 

 with a normal X and a lethal-bearing X chromosome. The former 

 are always normal in behavior, and the latter repeat in their descen- 

 dants the 2 : 1 sex-ratio. 



Whether a female bearing the same lethal twice (i. e., one homozygous 

 for a given lethal) would die, can not be stated, for no such females are 

 obtainable, because the lethal males, which alone could bring about 



