156 



Linkage and Crossing Over 



These terms have no more significance today than to indicate 

 the manner in which the genes were combined on the chromo- 

 somes of the Fi generation, but they are terms of convenience 

 and are frequently encountered in genetic hterature. At one time 

 they referred to concepts which have long been discarded. Genes 

 of maize in the coupling and repulsion phases are shown in 

 Fig. 51. 



Fig. 51. Linkage between colored and shrunken in maize. Left, the 

 coupling phase resulting from the cross C Sh / c shX c sh / csh and show- 

 ing a preponderance of colored, nonshrunken and noncolored, shrunken 

 types. Right, the repulsion phase resulting from the cross C sh/ cShX 

 csh / csh and showing a preponderance of the colored, shrunken and 

 noncolored, nonshrunken types. (Photographs by Dr. W. Brooks Hamil- 

 ton.) 



