96 HEREDITY AND SEX 



B or D and C are combined. The prediction has been 

 fulfilled so many times and in so many ways that we 

 feel some assurance that we have discovered here a 

 working hypothesis of considerable interest. If the 

 hypothesis becomes established, it will enable us to 

 analyze the structure of the chromosomes themselves 

 in the sense that we can determine the relative location 

 of factors in the chromosomes. If, as seems not 

 improbable, the chromosomes are the most important 

 element in Mendelian inheritance, the determination 

 of the linear series of factors contained in them becomes 

 a matter of great theoretical interest ; for we gain 

 further insight into the composition of the material 

 on which heredity itself depends. 



There is a corollary to this explanation of crossing 

 over that has a very direct bearing on the results. In 

 the male there is only one X chromosome present. 

 Hence crossing over is impossible. The experimental 

 results show that no crossing over takes place for 

 sex-linked factors in the male of drosophila. 



Other factors, however, lie in other chromosomes. 

 In these cases the chromosomes exist in pairs in the 

 male as well as in the female. Does crossing over 

 occur here in both sexes? Let me illustrate this by 

 an example. In drosophila the factor for black 

 body color and the factor that gives vestigial wings 

 lie in the same chromosome, which we may call the 

 second chromosome. If a black, long-winged female 

 is crossed with a gray vestigial male, all the offspring 

 will be gray in color and have long wings, since these 

 are the dominant characters. If these Fi flies are 

 inbred, the following classes will appear; 



