SEX-LINKED CHARACTERS IN DROSOPHILA. 19! 



DISCUSSION OF THE PROBLEM IN THE LlGHT OF THE 

 EXPERIMENTS. 



In the above experiments we find Morgan's observations 

 verified. 



1. A coupling occurs in the F 2 generation in which the coupled 

 factors are those that were associated in the grandparents. 



2. This coupling is by no means complete. 



3. The females of any class as a rule outnumber the males. 



4. The classes which lack the factor for red eye or normal 

 body color contain a smaller number of individuals than those 

 which possess this factor, and the class lacking both of them is 

 the smallest of all. 



Perhaps the last two conclusions may be stated: The vitality 

 of any class of individuals is increased according to the number 

 they possess of factors found in the normal (NR) female. On 

 this assumption, the reason why the NR males are less numerous 

 than the NR females would be accounted for by their having the 

 simplex rather than the duplex gametic constitution. Perhaps 

 an individual has an added vitality even through the possession 

 of two rather than one of the factors for sex, as in the case of the 

 YW females as opposed to the YW males. I do not desire to 

 urge the case, particularly since I understand Morgan is con- 

 ducting investigations that will throw more light on the subject. 



The particular problem presented by these experiments is to 

 explain why the coupling occurs but is not complete. This 

 problem perhaps can be finally settled only by the cytologist. 



Morgan's suggestion, based on the observations of Jannsens, 

 on the twisting of the chromosomes, I understand as follows. 

 The factors through the influence of which any particular char- 

 acter appears in an individual, are represented in the chromo- 

 somes by material particles which have always the same relative 

 linear position. " When the parental pairs (in the heterozygote) 

 conjugate, like regions will stand opposed. There is good evi- 

 dence to support the view that during the strepsinema stage, 

 homologous chromosomes twist around each other, but when 

 the chromosomes separate (split), the split is in a single plane." 1 



1 Morgan, in Science, Sept. 22, 1911, p. 384. 



