LINKAGE, CROSSING-OVER, AND THE GERM PLASM 291 



of course, also possible that the twisted chromosomes do not break 

 and reunite where they cross, and if they do not, then when they begin 

 to separate they simply pull apart irrespective of the side on which they 

 lie. When this occurs each chromosome remains intact and no 

 crossing-over takes place. 



"The chance that such a process of crossing-over will occur between 

 any two given points on the chromosome should obviously be greater 

 the greater the distance between those points. If then the Mendelian 

 factors he along the chromosomes, the amount of crossing-over be- 



a 



w 



B 



C 



D 



Fig. 73. — B and C illustrate Morgan's idea of the linear arrangement of the 

 genes in the chromosomes. A and D show how the composition of the chromo- 

 somes is supposed to change as the result of the crossover. On the right, a pair 

 of chromosomes, a, before; b, during; and c, after a double crossover. {After 



Morgan.) 



tween any two of them will depend upon their distance apart. Should 

 two points he near together a cross-over will only rarely occur between 

 them; if they he farther apart the chance of such a cross-over taking 

 place at some point between them will be greater. From this point 

 of view the percentage of crossing-over is an expression of the 'dis- 

 tance' of the factors from each other." 



CHROMOSOME MAPS INDICATING THE ARRANGEMENT OF 

 MENDELIAN FACTORS OR GENES IN THE CHROMOSOMES 



By making use of the fertile idea explained in the last paragraph, 

 that the percentage of crossing-over between any two factors indicates 

 their relative distances apart, it was possible to map out the rela- 



