GENES AND CHARACTERS 



307 



Linkage is oasily explained by the chromosome theory of hered- 

 ity, for organisms may have many dozens or hundreds of unit 



Fig. 175. — Back-cross of Fi female (out of black by vestigial) with black 

 vestigial male. B indicates normal color, b black, V normal wings and v 

 vestigial. An illustration of crossing over. (From Morgan et al, Mechan- 

 ism of Mendelian Heredity, with the permission of Henry Holt and Com- 

 pany.) 



characters but only a few chromosomes. Each chromosome in 

 such cases must necessarily bear the genes of many different 

 characters, and since the chromosome and not the gene is the 



