32 



SEX-LINKED INHERITANCE IN DROSOPHILA. 



The explanation of this mode of transmission became clear when it 

 was found that the cause of the death of half of the males was a 

 particular factor that had as definite a locus in the X chromosome as 

 have other sex-linked factors (Morgan, I9I2<?). Morgan mated females 

 (from the stock sent to him by Miss Rawls) to white-eyed males. 

 Half of the females, as expected, gave 2: i sex-ratios, and daughters 

 from these were again mated to white males. Here once more half of 

 the daughters gave 2 : i sex-ratios, but in such cases the sons were 

 nearly all white-eyed and only rarely a red-eyed son appeared, when 

 under ordinary circumstances there should be just as many red sons 

 as white sons. The total output for 11 such females was as follows 

 (Morgan, 1914^): white?, 457; red 9 , 433; white d^, 370; redcf, 2. 

 It is evident from these data that there must be present in the sex-chro- 

 mosome a gen that causes the death of every male that receives this 

 chromosome, and that this lethal factor lies very close to the factor for 

 white eyes. The Hnkage of this lethal (now called lethal i) to various 

 other sex-linked gens was determined (Morgan 1914^), and is summa- 

 rized in table 5. On the basis of these data it is found that the gen 

 lethal I lies 0.4 unit to the left of white, or at 0.7. 



Table 5. — Summary of linkage data upon lethal i , from Morgan, 1914 h, pp. 81-Q2. 



LETHAL la. 



In the second generation of the flies bred by Miss Rawls, one female 

 gave (March 191 2) only 3 sons, although she gave 312 daughters. It 

 was not known for some time (see lethals 3 and 3a) what was the 

 cause of this extreme rarity of sons. It is now apparent, however, 

 that this mother carried lethal i in one X and in the other X a new 

 lethal which had arisen by mutation. The new lethal was very close 

 to lethal I, as shown by the rarity of the surviving sons, which are 

 cross-overs between lethal i and the new lethal that we may call lethal 

 la. There is another class of cross-overs, namely, those which have 

 lethal I and get lethal la by crossing-over. These doubly lethal males 

 must also die, but since they are theoretically as numerous as the males 

 (3) free from both lethals, we must double this number (3X2) to get 

 the total number of cross-overs. There were 312 daughters, but as 

 the sons are normally about 96 per cent of the number of the females. 



