112 



EVOLUTION AND GENETICS 



of four kinds. Two of these are like the grand- 

 parents, one black-vestigial, the other normal-long, 

 but the other two kinds have, as it were, interchanged 



Fig. 49. — Diagram to illustrate crossing-over. The two 

 mutant characters, black and vestigial, are linked, as are 

 their normal allelomorphs, gray and long. A black vestigial 

 male is mated to a gray long female. The F^ female is 

 back -crossed to the double recessive type, black vestigial. 

 Four kinds of offspring are produced. 



