Chapter IX 



CROSSIXG-OVER 



If the linkage were never broken we should expect 

 to find that groups of characters would be inherited 

 together. There would be as many such groups of 

 characters as there are pairs of chromosomes. To a 

 certain extent this is true, but the study of the inher- 

 itance of two or more characters in the same linkage 

 group has revealed a further fact of great interest, 

 namely, that there takes place an interchange at times 

 between the two members of the same linkage group, 

 and, it may be added, only between members of the 

 same linkage group and never between different 

 linkage groups. This interchange gives rise to a new 

 phenomenon in inheritance that is called crossing- 

 over, which may be illustrated by a few typical ex- 

 amples from Drosophila. 



There are two mutant characters, black body 

 color and vestigial w^ngs, whose genes lie in the sec- 

 ond chromosome. If a fly having these two charac- 

 ters is crossed to a wild type fly with normal color 

 and long wings {jig. 49) the offspring are like wild 

 type flies, because normal dominates black and long 

 wing dominates vestigial wing. 



If one of the daughters (i^i) from this cross is 

 now mated to a male with black color and vestigial 

 wings (both recessive characters) the offsj^ring are 



