80 T. H. MORGAN AND ELETH CATTELL 



somes (X) when present, or absent from the chromosomes when 

 the absence of the factor is involved. In the male-producing 

 sperm, where no X is present, the sex-linked characters are always 

 absent. A corollary to this point of view involves the crucial 

 point of the chromosome theory of linkage (Morgan^ '11). In 

 the female two X-chromosomes are always present. If one of 

 them contains two of the factors in question, such as the factor 

 for red^ (R) and that for long wings (L), and the other X-chromo- 

 some contains no factor for red (absence of red or W) and short 

 wings (or S) it is possible for interchange (or 'breaking,' or 'cross- 

 ing-over') between RL and WS to take place. How often this 

 may occur depends on the strength of the linkage of the factors 

 involved (which one of us has tried to interpret as due to their 

 position in the chromosomes). But in the male, on the other 

 hand, no such interchange is theoretically possible, and the results 

 show that none occurs. It is this simple and consistent relation 

 that gives 'point' to the chromosomal interpretation. 



In order to have a basis for interpreting the more compli- 

 cated cases we will first give the results of an experiment where 

 only two contrasted characters, black and yellow, are involved. 

 In this experiment no question of linkage is involved, since the 

 factor for black and that for yellow are not in the same chromo- 

 some. The experiment also gives information showing the via- 

 bility of the gray, black, yellow and brown flies. 



When black females were mated to yellow males all the off- 

 spring were gray (N). These inbred gave the following results: 



LRB 9 by LRY d^ 



LRN 9 = 50 

 ^' LRN d^ = 60 



^ In reality the presence of C gives red and its absence c white 



